SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF CASH FLOWS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders	$ 1,106.4	$ 230.4	$ 509.2
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(361.5)	(184.9)	(434.2)
Amortization of restricted share awards	18.5	16.3	15.3
Net other operating activities	89.6	(42.1)	(5.5)
Net cash provided from (used for) operations	550.5	586.8	404.1
Cash flows from investing activities:
Net change in short-term investments	(872.2)	548.4	(495.8)
Release of cash (pre-funding) of investment purchases	(30.0)	0.0	70.0
Net cash provided from (used for) investing activities	(666.9)	(447.0)	(543.9)
Cash flows from financing activities:
Repurchases and retirement of common shares	(202.6)	(7.9)	(32.7)
Cash dividends paid to common shareholders	(2.6)	(2.5)	(2.6)
Net cash provided from (used for) financing activities	35.6	4.2	7.2
Cash balance at beginning of year	266.4	122.4	255.0
Cash balance at end of year	185.6	266.4	122.4
Net change in (receivables) payables to subsidiaries	8.1	(14.4)	8.4
Purchases of fixed maturity investments	1,235.2	1,677.5	592.7
WM Hinson
Cash flows from financing activities:
Distributions, cash	875.0	47.0
Repayments of debt in short-term investments			61.7
Proceeds from borrowings	90.0	208.9	20.0
Repayments of principal debt	183.9	190.0	42.4
Repayments of debt in investments			107.6
Repayments of debt in fixed maturity investments			45.9
WM Webster
Cash flows from financing activities:
Distributions, cash		7.5
Distributions, short-term investments		4.9
WM Madison
Cash flows from financing activities:
Distributions, cash		$ 2.1
Ownership subsidiary		3.50%
WM Taylor
Cash flows from financing activities:
Distributions, cash		$ 300.0
Ownership subsidiary		94.20%
White Mountain Investments Luxembourg
Cash flows from financing activities:
Purchases of fixed maturity investments		$ 8.0
Proceeds from borrowings	235.0		105.0
HG Global
Cash flows from financing activities:
Proceeds from borrowings	4.0	15.2	7.5
Repayments of principal debt	4.0	19.2	9.5
Subsidiary - White Mountains Investments Bermuda
Cash flows from financing activities:
Distributions, cash	35.0	4.3
Contributions, short-term investments and cash	30.0
Repayments of debt in short-term investments	29.9
Contributions, cash	0.1
Distributions, short-term investments		18.7
Subsidiary - White Mountain Adams, Inc.
Cash flows from financing activities:
Contributions, short-term investments and cash		319.5
Repayments of debt in short-term investments		17.2
Contributions, cash		0.2
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders	1,106.4	230.4	509.2
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(15.8)	(50.5)	(78.9)
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax	(1,159.3)	(232.3)	(489.2)
Net other non-cash reconciling items	(1.1)	(1.1)	(2.6)
Distributions of accumulated earnings from consolidated subsidiaries	29.9	24.2	17.3
Net other operating activities	14.2	(17.1)	9.2
Net cash provided from (used for) operations	(7.2)	(30.1)	(19.7)
Cash flows from investing activities:
Net change in short-term investments	41.1	133.9	(63.8)
Purchases of investment securities	0.0	(107.9)	(125.0)
Maturities, calls, paydowns and sales of investment securities	305.6	100.0	335.4
Purchases of investment securities from subsidiaries	(8.0)	(12.0)	0.0
Release of cash (pre-funding) of investment purchases	0.0	0.0	70.0
Net issuance of debt (to) from subsidiaries	(279.0)	(224.1)	(132.5)
Net repayment of debt to (from) subsidiaries	187.9	209.2	31.4
Net distributions from (contributions to) subsidiaries	(34.9)	(58.8)	(60.3)
Net cash provided from (used for) investing activities	212.7	40.3	55.2
Cash flows from financing activities:
Repurchases and retirement of common shares	(202.6)	(7.9)	(32.7)
Cash dividends paid to common shareholders	(2.6)	(2.5)	(2.6)
Net cash provided from (used for) financing activities	(205.2)	(10.4)	(35.3)
Net change in cash during the year	0.3	(0.2)	0.2
Cash balance at beginning of year	0.1	0.3	0.1
Cash balance at end of year	0.4	0.1	0.3
Noncash payment in short-term investments			10.0
Purchases of fixed maturity investments		12.0
Proceeds from borrowings	50.0
White Mountains Insurance Group Ltd. | WM Lafayette
Cash flows from financing activities:
Purchases of fixed maturity investments	8.0
White Mountains Insurance Group Ltd. | WM Pierce
Cash flows from financing activities:
Purchases of fixed maturity investments	$ 9.9
Subsidiary - WM Birkdale
Cash flows from financing activities:
Repayments of debt in short-term investments			36.9
Purchases of fixed maturity investments		$ 4.0
Repayments of principal debt			20.5
Repayments of debt in investments			73.8
Repayments of debt in fixed maturity investments			$ 36.9