SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF CASH FLOWS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss)	$ 284.1	$ 580.9	$ 712.4
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(184.9)	(434.2)	191.5
Net (income) loss from discontinued operation, net of tax - NSM Group			(902.5)
Amortization of restricted share awards	16.3	15.3	14.0
Net other operating activities	(50.5)	(3.9)	80.8
Net cash provided from (used for) operations	586.8	404.1	365.0
Cash flows from investing activities:
Net change in short-term investments	548.4	(495.8)	(455.1)
Release of cash (pre-funding) for ILS funds managed by Elementum	0.0	70.0	(70.0)
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.0, $0.0 and $0.5	0.0	17.3	1,392.0
Cash sold in subsidiary transaction	4.2	0.0	0.0
Net cash provided from (used for) investing activities	(447.0)	(543.9)	115.5
Cash flows from financing activities:
Repurchases and retirement of common shares	(7.9)	(32.7)	(615.8)
Cash dividends paid to common shareholders	(2.5)	(2.6)	(3.0)
Net cash provided from (used for) financing activities	4.2	7.2	(344.9)
Cash balance at beginning of year	122.4	255.0	147.7
Cash balance at end of year	266.4	122.4	255.0
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Common equity securities, at fair value	650.0	538.4
Other long-term investments	2,150.2	1,998.2
Net change in (receivables) payables to subsidiaries	(14.4)	8.4	3.3
Purchases of fixed maturity investments	1,677.5	592.7	585.5
White Mountains Adams, Inc.
Cash flows from financing activities:
Distributions, cash			25.0
Bridge Holdings, Ltd.
Cash flows from financing activities:
Total White Mountains’s common shareholders’ equity			3,540.6
Intercompany note receivable			76.4
Investments in consolidated subsidiaries			2,003.6
Fixed maturity investments, at fair value			28.6
Common equity securities, at fair value			8.1
Other long-term investments			52.2
Short-term investments, at fair value			1,358.7
Other assets			6.0
Proceeds from borrowings			69.0
WM Hinson
Cash flows from financing activities:
Distributions, cash	47.0
Repayments of debt in short-term investments		61.7
Proceeds from borrowings	208.9	20.0
Repayments of principal debt	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%
Purchases of fixed maturity investments			14.0
WM Taylor
Cash flows from financing activities:
Distributions, cash	$ 300.0
Ownership subsidiary	94.20%
HG Global
Cash flows from financing activities:
Proceeds from borrowings	$ 15.2	7.5
Repayments of principal debt	19.2	9.5
White Mountain Investments Luxembourg
Cash flows from financing activities:
Proceeds from borrowings		105.0
Purchases of fixed maturity investments	8.0
Subsidiary - Bridge Holdings, Ltd.
Charges (credits) to reconcile net income to net cash from operations:
Accumulated earnings distributed from subsidiary in cash			7.0
Cash flows from financing activities:
Repayments of principal debt			15.0
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss)	230.4	509.2	792.8
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(50.5)	(78.9)	5.6
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax	(232.3)	(489.2)	42.0
Net (income) loss from discontinued operation, net of tax - NSM Group	0.0	0.0	(16.4)
Other non-cash reconciling items	(1.1)	(2.6)	(7.0)
Accumulated earnings distributed from subsidiary in cash	24.2	17.3	7.0
Net other operating activities	(17.1)	9.2	17.2
Net cash provided from (used for) operations	(30.1)	(19.7)	(31.6)
Cash flows from investing activities:
Net change in short-term investments	133.9	(63.8)	1,165.6
Purchases of investment securities	(107.9)	(125.0)	(359.2)
Maturities, calls, paydowns and sales of investment securities	100.0	335.4	15.7
Purchases of investment securities from subsidiaries (7)	(12.0)	0.0	(14.0)
Release of cash (pre-funding) for ILS funds managed by Elementum	0.0	70.0	(70.0)
Net issuance of debt (to) from subsidiaries	(224.1)	(132.5)	(142.0)
Net repayment of debt to (from) subsidiaries	209.2	31.4	(15.0)
Net distributions from (contributions to) subsidiaries	(58.8)	(60.3)	49.3
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.0, $0.0 and $0.5	0.0	0.0	19.5
Cash sold in subsidiary transaction	0.0	0.0	0.5
Net cash provided from (used for) investing activities	40.3	55.2	649.9
Cash flows from financing activities:
Repurchases and retirement of common shares	(7.9)	(32.7)	(615.8)
Cash dividends paid to common shareholders	(2.5)	(2.6)	(3.0)
Net cash provided from (used for) financing activities	(10.4)	(35.3)	(618.8)
Net change in cash during the year	(0.2)	0.2	(0.5)
Cash balance at beginning of year	0.3	0.1	0.6
Cash balance at end of year	0.1	0.3	0.1
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Investments in consolidated subsidiaries	3,944.5	3,666.9
Fixed maturity investments, at fair value	70.0	0.0
Common equity securities, at fair value	234.7	147.8
Other long-term investments	74.0	160.5
Short-term investments, at fair value	62.4	198.8
Other assets	4.6	4.0
Noncash payment in short-term investments		10.0
Purchases of fixed maturity investments	12.0
Subsidiary - White Mountains Investments Bermuda
Cash flows from financing activities:
Distributions, cash	4.3		51.0
Distributions, short-term investments	18.7
Subsidiary - White Mountain Adams, Inc.
Cash flows from financing activities:
Distributions, short-term investments			100.0
Contributions, short-term investments and cash	319.5
Repayments of debt in short-term investments	17.2
Contributions, cash	0.2
Subsidiary - HG Global
Cash flows from financing activities:
Contributions, cash			116.3
Proceeds from borrowings			6.0
Subsidiary - Ark Insurance Holdings Limited
Cash flows from financing activities:
Contributions, cash			7.7
Subsidiary - PSC Holdings, Ltd.
Cash flows from financing activities:
Contributions, cash			1.3
Subsidiary - WM Birkdale
Cash flows from financing activities:
Repayments of debt in short-term investments		36.9
Repayments of principal debt		20.5
Repayments of debt in investments		73.8
Repayments of debt in fixed maturity investments		$ 36.9
Purchases of fixed maturity investments	$ 4.0
Subsidiary - White Mountain Hinson Bermuda
Cash flows from financing activities:
Proceeds from borrowings			$ 205.0