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

SecurityWTM / White Mountains Insurance Group, Ltd.
InstitutionNorthern Trust Corp
Latest Disclosed Ownership15,495 shares
Latest Disclosed Value $ 34,041,895
Northern Trust Corp reports 7.03% decrease in ownership of WTM / White Mountains Insurance Group, Ltd.

On May 14, 2026 - Northern Trust Corp filed a 13F-HR form disclosing ownership of 15,495 shares of White Mountains Insurance Group, Ltd. (US:WTM) valued at $34,041,895 USD as of March 31, 2026. The entity filed a previous 13F-HR on February 17, 2026 disclosing 16,667 shares of White Mountains Insurance Group, Ltd.. This represents a change in shares of -7.03% during the quarter. The current value of the position is $31,230,792 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)
2026-05-14 2026-03-31 13F WHITE MTNS INS GROUP COM G9618E107 15,495 -1,172 -7.03 34,042 -1.71 0.0009
2026-02-17 2025-12-31 13F WHITE MTNS INS GROUP COM G9618E107 16,667 -1,090 -6.14 34,635 16.69 0.0044
2025-11-14 2025-09-30 13F WHITE MTNS INS GROUP COM G9618E107 17,757 352 2.02 29,681 -5.03 0.0038
2025-08-13 2025-06-30 13F WHITE MTNS INS GROUP COM G9618E107 17,405 -817 -4.48 31,255 -10.94 0.0043
2025-05-13 2025-03-31 13F WHITE MTNS INS GROUP COM G9618E107 18,222 151 0.84 35,092 -0.16 0.0052
2025-02-14 2024-12-31 13F WHITE MTNS INS GROUP COM G9618E107 18,071 1,957 12.14 35,149 28.60 0.0050
2024-11-13 2024-09-30 13F WHITE MTNS INS GROUP COM G9618E107 16,114 172 1.08 27,333 -5.66 0.0045
2024-08-14 2024-06-30 13F WHITE MTNS INS GROUP COM G9618E107 15,942 1,346 9.22 28,974 10.63 0.0049
2024-05-14 2024-03-31 13F WHITE MTNS INS GROUP COM G9618E107 14,596 -522 -3.45 26,190 15.11 0.0045
2024-02-13 2023-12-31 13F WHITE MTNS INS GROUP COM G9618E107 15,118 -601 -3.82 22,753 -3.22 0.0041
2023-11-13 2023-09-30 13F WHITE MTNS INS GROUP COM G9618E107 15,719 309 2.01 23,511 9.84 0.0046
2023-08-11 2023-06-30 13F WHITE MTNS INS GROUP COM G9618E107 15,410 289 1.91 21,403 2.76 0.0041
2023-05-15 2023-03-31 13F WHITE MTNS INS GROUP COM G9618E107 15,121 205 1.37 20,829 -1.27 0.0041
2023-02-13 2022-12-31 13F WHITE MTNS INS GROUP COM G9618E107 14,916 231 1.57 21,096 10.25 0.0044
2022-11-14 2022-09-30 13F WHITE MTNS INS GROUP COM G9618E107 14,685 -1,201 -7.56 19,134 -3.34 0.0044
2022-08-12 2022-06-30 13F WHITE MTNS INS GROUP COM G9618E107 15,886 326 2.10 19,795 11.96 0.0042
2022-05-13 2022-03-31 13F WHITE MTNS INS GROUP COM G9618E107 15,560 -5,856 -27.34 17,680 -18.57 0.0031
2022-02-08 2021-12-31 13F WHITE MTNS INS GROUP COM G9618E107 21,416 -141 -0.65 21,713 -5.83 0.0036
2021-11-15 2021-09-30 13F WHITE MTNS INS GROUP COM G9618E107 21,557 -6,026 -21.85 23,057 -27.19 0.0041
2021-08-13 2021-06-30 13F WHITE MTNS INS GROUP COM G9618E107 27,583 64 0.23 31,667 3.22 0.0056
2021-05-12 2021-03-31 13F WHITE MTNS INS GROUP COM G9618E107 27,519 -11,637 -29.72 30,680 -21.70 0.0057
2021-02-11 2020-12-31 13F WHITE MTNS INS GROUP COM G9618E107 39,156 470 1.21 39,182 30.02 0.0076
2020-11-16 2020-09-30 13F WHITE MTNS INS GROUP COM G9618E107 38,686 3,604 10.27 30,136 -3.26 0.0066
2020-08-14 2020-06-30 13F WHITE MTNS INS GROUP COM G9618E107 35,082 -6,981 -16.60 31,152 -18.62 0.0074
2020-05-14 2020-03-31 13F WHITE MTNS INS GROUP COM G9618E107 42,063 351 0.84 38,278 -17.73 0.0110
2020-02-14 2019-12-31 13F WHITE MTNS INS GROUP COM G9618E107 41,712 -9,442 -18.46 46,530 -15.78 0.0104
2019-11-13 2019-09-30 13F WHITE MTNS INS GROUP COM G9618E107 51,154 -1,636 -3.10 55,247 2.46 0.0132
2019-08-13 2019-06-30 13F WHITE MTNS INS GROUP COM G9618E107 52,790 -329 -0.62 53,923 9.69 0.0128
2019-05-13 2019-03-31 13F WHITE MTNS INS GROUP COM G9618E107 53,119 -167 -0.31 49,161 7.57 0.0121
2019-02-12 2018-12-31 13F WHITE MTNS INS GROUP COM G9618E107 53,286 -3,379 -5.96 45,703 -13.82 0.0128
2018-11-14 2018-09-30 13F WHITE MTNS INS GROUP COM G9618E107 56,665 -5,603 -9.00 53,031 -6.06 0.0130
2018-09-18 2018-06-30 13F/A-1 WHITE MTNS INS GROUP COM G9618E107 62,268 -1,691 -2.64 56,453 7.31 0.0143
2018-08-14 2018-06-30 13F WHITE MTNS INS GROUP COM G9618E107 62,507 -1,452 56,670
2018-05-09 2018-03-31 13F WHITE MTNS INS GROUP COM G9618E107 63,959 32,917 106.04 52,606 99.07 0.0137
2018-02-14 2017-12-31 13F WHITE MTNS INS GROUP COM G9618E107 31,042 -1,646 -5.04 26,426 -5.67 0.0067
2017-11-13 2017-09-30 13F WHITE MTNS INS GROUP COM G9618E107 32,688 -2,003 -5.77 28,013 -7.04 0.0073
2017-08-11 2017-06-30 13F WHITE MTNS INS GROUP COM G9618E107 34,691 -374 -1.07 30,134 -2.33 0.0084
2017-05-12 2017-03-31 13F WHITE MTNS INS GROUP COM G9618E107 35,065 4,036 13.01 30,853 18.93 0.0089
2017-02-13 2016-12-31 13F WHITE MTNS INS GROUP COM G9618E107 31,029 199 0.65 25,942 1.38 0.0080
2016-11-09 2016-09-30 13F WHITE MTNS INS GROUP COM G9618E107 30,830 802 2.67 25,590 1.21 0.0080
2016-08-12 2016-06-30 13F WHITE MTNS INS GROUP COM G9618E107 30,028 -318 -1.05 25,284 3.81 0.0082
2016-08-19 2016-03-31 13F/A-1 WHITE MTNS INS GROUP COM G9618E107 30,346 -503 -1.63 24,356 8.63 0.0081
2016-05-13 2016-03-31 13F WHITE MTNS INS GROUP COM G9618E107 30,346 24,356
2016-02-12 2015-12-31 13F White Mtns Ins Group COMM G9618E107 30,849 406 1.33 22,422 -1.44 0.0074
2015-11-12 2015-09-30 13F WHITE MTNS INS GROUP COM G9618E107 30,443 30,443 0.00 22,750 0.0076
2015-08-13 2015-06-30 13F WHITE MTNS INS GROUP COM G9618E107 0 -26,166 -100.00 0 -100.00
2015-05-14 2015-03-31 13F WHITE MTNS INS GROUP COM G9618E107 26,166 -1,383 -5.02 17,911 3.19 0.0054
2015-02-12 2014-12-31 13F WHITE MTNS INS GROUP COM G9618E107 27,549 -661 -2.34 17,358 -2.34 0.0052
2014-11-13 2014-09-30 13F WHITE MTNS INS GROUP COM G9618E107 28,210 1,971 7.51 17,774 11.33 0.0056
2014-08-14 2014-06-30 13F WHITE MTNS INS GROUP COM G9618E107 26,239 994 3.94 15,965 5.42 0.0050
2014-05-14 2014-03-31 13F WHITE MTNS INS GROUP COM G9618E107 25,245 -1,423 -5.34 15,144 -5.84 0.0048
2014-02-21 2013-12-31 13F/A-1 WHITE MTNS INS GROUP COM G9618E107 26,668 -36 -0.13 16,083 6.11 0.0052
2014-02-12 2013-12-31 13F WHITE MTNS INS GROUP COM G9618E107 26,668 16,083
2013-11-13 2013-09-30 13F WHITE MTNS INS GROUP COM G9618E107 26,704 1,526 6.06 15,157 4.70 0.0053
2013-08-09 2013-06-30 13F WHITE MTNS INS GROUP COM G9618E107 25,178 25,178 14,476 0.0052
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.