White Mountains Insurance Group, Ltd.
US ˙ NYSE ˙ BMG9618E1075

SecurityWTM / White Mountains Insurance Group, Ltd.
InstitutionState Street Corp
Latest Disclosed Ownership49,158 shares
Latest Disclosed Value $ 107,998,160
State Street Corp reports 2.97% decrease in ownership of WTM / White Mountains Insurance Group, Ltd.

On May 15, 2026 - State Street Corp filed a 13F-HR form disclosing ownership of 49,158 shares of White Mountains Insurance Group, Ltd. (US:WTM) valued at $107,998,160 USD as of March 31, 2026. The entity filed a previous 13F-HR on February 13, 2026 disclosing 50,663 shares of White Mountains Insurance Group, Ltd.. This represents a change in shares of -2.97% during the quarter. The current value of the position is $99,079,915 USD.

Institutional Ownership: 13F and NPORT Filings

The Security and Class in the table below are shown exactly as filed by the investor. We do our best to track continuity of investments through acquisitions, and this will be reflected in the table as changes in names. In addition to descriptive data, performance of the investment is shown over time. To calculate quarterly performance, we first calculate cost basis of the shares purchased during the quarter, then use that to calculate gross profit. Quarterly return is Gross Profit / Starting Portfolio value.

Note that cost basis is calculated and stored in thousands, so small quarterly changes in shares frequently result in a cost basis of zero.

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Disclosed Equity Positions (from 13F/NPORT Filings)
File
Date		 Effective
Date		Form Security Class ID Avg Price
Paid (Est)		 Reported
Shares		 ΔShares ΔShares
(%)		 Reported
Value
(x1000)		 ΔValue
(%)		 Alloc
(%)		 ΔAlloc ΔAlloc (%) Cost Basis
(x1000)		 Unrealized
Gain/Loss
(x1000)
2026-05-15 2026-03-31 13F WHITE MTNS INS GROUP COM G9618E107 49,158 -1,505 -2.97 107,998 2.58 0.0037
2026-02-13 2025-12-31 13F WHITE MTNS INS GROUP COM G9618E107 50,663 -3,781 -6.94 105,279 15.69 0.0035
2025-11-14 2025-09-30 13F WHITE MTNS INS GROUP COM G9618E107 54,444 -2,797 -4.89 91,004 -11.46 0.0032
2025-08-14 2025-06-30 13F WHITE MTNS INS GROUP COM G9618E107 57,241 2,297 4.18 102,789 -2.86 0.0039
2025-05-15 2025-03-31 13F WHITE MTNS INS GROUP COM G9618E107 54,944 59 0.11 105,812 -0.88 0.0044
2025-02-14 2024-12-31 13F WHITE MTNS INS GROUP COM G9618E107 54,885 8 0.01 106,755 14.69 0.0042
2024-11-14 2024-09-30 13F WHITE MTNS INS GROUP COM G9618E107 54,877 2,714 5.20 93,082 -1.82 0.0038
2024-08-14 2024-06-30 13F WHITE MTNS INS GROUP COM G9618E107 52,163 3,543 7.29 94,804 8.67 0.0041
2024-05-15 2024-03-31 13F WHITE MTNS INS GROUP COM G9618E107 48,620 -1,585 -3.16 87,239 15.46 0.0039
2024-02-14 2023-12-31 13F WHITE MTNS INS GROUP COM G9618E107 50,205 3,240 6.90 75,559 7.56 0.0037
2023-11-14 2023-09-30 13F WHITE MTNS INS GROUP COM G9618E107 46,965 -664 -1.39 70,245 6.19 0.0039
2023-08-14 2023-06-30 13F WHITE MTNS INS GROUP COM G9618E107 47,629 -244 -0.51 66,152 0.32 0.0035
2023-05-15 2023-03-31 13F WHITE MTNS INS GROUP COM G9618E107 47,873 -2,476 -4.92 65,945 -7.40 0.0037
2023-05-15 2022-12-31 13F/A-99 WHITE MTNS INS GROUP COM G9618E107 50,349 -1,932 -3.70 71,210 4.53 0.0042
2023-05-15 2022-12-31 13F WHITE MTNS INS GROUP COM G9618E107 50,349 -1,932 71,210 0.0042
2022-11-15 2022-09-30 13F WHITE MTNS INS GROUP COM G9618E107 52,281 -1,998 -3.68 68,123 0.72 0.0043
2022-08-15 2022-06-30 13F WHITE MTNS INS GROUP COM G9618E107 54,279 -1,545 -2.77 67,639 6.64 0.0041
2022-05-16 2022-03-31 13F WHITE MTNS INS GROUP COM G9618E107 55,824 2,365 4.42 63,429 17.02 0.0031
2022-02-14 2021-12-31 13F WHITE MTNS INS GROUP COM G9618E107 53,459 -2,459 -4.40 54,202 -9.38 0.0026
2021-11-15 2021-09-30 13F WHITE MTNS INS GROUP COM G9618E107 55,918 -4,866 -8.01 59,810 -14.29 0.0032
2021-08-16 2021-06-30 13F WHITE MTNS INS GROUP COM G9618E107 60,784 12,904 26.95 69,782 30.72 0.0037
2021-07-09 2021-03-31 13F/A-1 WHITE MTNS INS GROUP COM G9618E107 47,880 -1,204 -2.45 53,381 8.68 0.0031
2021-05-17 2021-03-31 13F WHITE MTNS INS GROUP COM G9618E107 47,880 -1,204 53,381 0.0031
2021-02-16 2020-12-31 13F WHITE MTNS INS GROUP COM G9618E107 49,084 -2,391 -4.64 49,116 22.49 0.0030
2020-11-10 2020-09-30 13F WHITE MTNS INS GROUP COM G9618E107 51,475 -6,365 -11.00 40,099 -21.93 0.0027
2020-08-14 2020-06-30 13F WHITE MTNS INS GROUP COM G9618E107 57,840 -1,512 -2.55 51,360 -4.91 0.0037
2020-06-19 2020-03-31 13F/A-1 WHITE MTNS INS GROUP COM G9618E107 59,352 -2,757 -4.44 54,010 -22.04 0.0047
2020-05-11 2020-03-31 13F WHITE MTNS INS GROUP COM G9618E107 59,352 -2,757 54,010 4,743.1655
2020-02-06 2019-12-31 13F WHITE MTNS INS GROUP COM G9618E107 62,109 423 0.69 69,283 4.00 0.0047
2019-11-14 2019-09-30 13F WHITE MTNS INS GROUP COM G9618E107 61,686 5,843 10.46 66,621 16.79 0.0050
2019-08-14 2019-06-30 13F WHITE MTNS INS GROUP COM G9618E107 55,843 -909 -1.60 57,041 8.60 0.0043
2019-05-15 2019-03-31 13F WHITE MTNS INS GROUP COM G9618E107 56,752 2,753 5.10 52,523 13.41 0.0041
2019-02-12 2018-12-31 13F WHITE MTNS INS GROUP COM G9618E107 53,999 -1,041 -1.89 46,314 -10.09 0.0042
2018-11-09 2018-09-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 55,040 -4,881 -8.15 51,512 -5.17 0.0040
2018-08-14 2018-06-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 59,921 -7,290 -10.85 54,323 -1.74 0.0045
2018-05-15 2018-03-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 67,211 -6,744 -9.12 55,286 -12.18 0.0047
2018-02-14 2017-12-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 73,955 2,967 4.18 62,952 3.46 0.0051
2017-11-14 2017-09-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 70,988 -1,291 -1.79 60,845 -3.09 0.0052
2017-08-14 2017-06-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 72,279 11,953 19.81 62,785 18.28 0.0056
2017-05-15 2017-03-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 60,326 492 0.82 53,083 6.11 0.0047
2017-02-09 2016-12-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 59,834 -5,795 -8.83 50,026 -8.16 0.0047
2017-01-13 2016-09-30 13F/A-1 WHITE MTNS INS GROUP Common equity shares G9618E107 65,629 5,713 9.54 54,472 7.97 0.0055
2016-11-14 2016-09-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 65,629 54,472
2016-08-12 2016-06-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 59,916 -7,047 -10.52 50,450 -6.13 0.0054
2016-05-20 2016-03-31 13F/A-1 WHITE MTNS INS GROUP Common equity shares G9618E107 66,963 768 1.16 53,746 11.71 0.0058
2016-05-12 2016-03-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 66,963 53,746
2016-02-16 2015-12-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 66,195 5,181 8.49 48,111 5.53 0.0053
2015-11-16 2015-09-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 61,014 -4,496 -6.86 45,592 6.28 0.0053
2015-08-13 2015-06-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 65,510 -6,378 -8.87 42,898 -12.82 0.0046
2015-05-15 2015-03-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 71,888 -1,830 -2.48 49,205 5.93 0.0050
2015-02-11 2014-12-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 73,718 -432 -0.58 46,452 -0.57 0.0046
2014-11-12 2014-09-30 13F/A-1 WHITE MTNS INS GROUP Common equity shares G9618E107 74,150 -691 -0.92 46,720 2.59 0.0050
2014-11-10 2014-09-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 74,150 46,720
2014-08-12 2014-06-30 13F WHITE MTNS INS GROUP Common equity shares G9618E107 74,841 -1,040 -1.37 45,540 0.03 0.0049
2014-05-15 2014-03-31 13F/A-1 WHITE MTNS INS GROUP Common equity shares G9618E107 75,881 -6,175 -7.53 45,525 -8.01 0.0052
2014-05-13 2014-03-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 75,981 45,585
2014-02-06 2013-12-31 13F/A-1 WHITE MTNS INS GROUP Common equity shares G9618E107 82,056 -83 -0.10 49,487 6.14 0.0055
2014-02-06 2013-12-31 13F WHITE MTNS INS GROUP Common equity shares G9618E107 83,784 43,157
2013-11-12 2013-09-30 13F WHITE MTNS INS GROUP Common Stock G9618E107 82,139 -166 -0.20 46,626 -1.45 0.0057
2013-08-08 2013-06-30 13F WHITE MTNS INS GROUP Common Stock G9618E107 82,305 82,305 47,314 0.0062
Legend
Shares
The total number of shares held by the institution at the end of the reporting period (the effective date). This is provided in the filing.
Value
The total value of the shares as of the effective date. This is provided in the filing.
Avg. Share Price
The weighted average share price of the shares held by the institution. We use FIFO accounting to determine this price
Class
This is the security class as indicated by the filer. There are a variety of values for this field. Common values include "EC" = equity common, "EP" - equity preferred
Allocation (%)
The percent of the institution's portfolio that this position represents. This is provided in NPORT filings. For 13F filings, we calculate it.
Change in Allocation
The change in allocation from the prior reporting period. This is calculated as current allocation - prior allocation.
Change in Allocation (%)
The percent change in allocation from the prior reporting period. This is calculated as (current allocation - prior allocation) / prior allocation.
Cost Basis
The total cost of the shares held. This is calculated as reported shares * avg. share price
Unrealized Gain/Loss
This is the market value of the position as of the effective date minus the cost basis.