SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF CASH FLOWS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss)	$ 580.9	$ 712.4	$ (321.9)
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(434.2)	191.5	246.1
Net (income) loss from discontinued operation, net of tax - NSM Group		(902.5)	2.9
Net other operating activities	(3.9)	80.8	10.7
Net cash provided from (used for) operations	404.1	365.0	38.6
Cash flows from investing activities:
Net change in short-term investments	(495.8)	(455.1)	59.8
Release of cash (pre-funding) for ILS funds managed by Elementum	70.0	(70.0)	0.0
Proceeds from the sale of business, net of cash sold	17.3	1,392.0	0.0
Cash sold in subsidiary transaction	0.8	144.4	0.0
Net cash provided from (used for) investing activities	(543.9)	115.5	(208.3)
Cash flows from financing activities:
Repurchases and retirement of common shares	(32.7)	(615.8)	(107.5)
Cash dividends paid to common shareholders	(2.6)	(3.0)	(3.1)
Net cash provided from (used for) financing activities – continuing operations	7.2	(344.9)	217.6
Cash balance at beginning of year	255.0	147.7	84.6
Cash balance at end of year	122.4	255.0	147.7
Amortization of restricted share awards	15.3	14.0	14.7
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Common equity securities, at fair value	538.4	668.4
Other long-term investments	1,998.2	1,488.0
Net change in (receivables) payables to subsidiaries	8.4	3.3	(6.5)
WM Lincoln
Cash flows from financing activities:
Investments in consolidated subsidiaries			212.6
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss)	509.2	792.8	(275.4)
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	(78.9)	5.6	0.1
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax	(489.2)	42.0	232.3
Net (income) loss from discontinued operation, net of tax - NSM Group	0.0	(16.4)	22.6
Other non-cash reconciling items	12.7	7.0	16.6
Accumulated earnings distributed from subsidiary in cash	17.3	7.0	0.0
Net other operating activities	9.2	17.2	(5.7)
Net cash provided from (used for) operations	(19.7)	(31.6)	(28.2)
Cash flows from investing activities:
Net change in short-term investments	(63.8)	1,165.6	17.7
Purchases of investment securities	(125.0)	(359.2)	0.0
Sales and maturities of investment securities	335.4	15.7	0.0
Purchases of investment securities from subsidiaries	0.0	(14.0)	(26.4)
Sales of investment securities to subsidiaries	0.0	0.0	36.4
Release of cash (pre-funding) for ILS funds managed by Elementum	70.0	(70.0)	0.0
Net issuance of debt (to) from subsidiaries	(132.5)	(142.0)	94.0
Net repayment of debt to (from) subsidiaries	31.4	(15.0)	0.0
Net distributions from (contributions to) subsidiaries	(60.3)	49.3	17.0
Proceeds from the sale of business, net of cash sold	0.0	19.5	0.0
Cash sold in subsidiary transaction	0.0	0.5	0.0
Net cash provided from (used for) investing activities	55.2	649.9	138.7
Cash flows from financing activities:
Repurchases and retirement of common shares	(32.7)	(615.8)	(107.5)
Cash dividends paid to common shareholders	(2.6)	(3.0)	(3.1)
Net cash provided from (used for) financing activities – continuing operations	(35.3)	(618.8)	(110.6)
Net change in cash during the year	0.2	(0.5)	(0.1)
Cash balance at beginning of year	0.1	0.6	0.7
Cash balance at end of year	0.3	0.1	0.6
Total White Mountains’s common shareholders’ equity	4,240.5	3,746.9
Investments in consolidated subsidiaries	3,666.9	3,027.2
Fixed maturity investments, at fair value	0.0	49.2
Common equity securities, at fair value	147.8	342.8
Other long-term investments	160.5	49.3
Short-term investments, at fair value	198.8	105.2
Other assets	4.0	75.3
Noncash payment in short-term investments	10.0
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
Contributions			12.6
Contributions, short-term investments and cash			18.0
Contributions, short-term investments			17.9
Contributions, cash			0.1
Subsidiary - White Mountain Adams, Inc.
Cash flows from financing activities:
Distributions			42.7
Distributions, short-term investments		100.0	37.1
Distributions, cash			5.6
Subsidiary - White Mountains Investments Bermuda
Cash flows from financing activities:
Contributions			19.7
Contributions, short-term investments			19.1
Distributions, cash		51.0
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 - White Mountain Hinson Bermuda
Cash flows from financing activities:
Contributions, cash			0.6
Proceeds from borrowings		205.0
WM Birkdale
Cash flows from financing activities:
Repayments of principal debt	20.5
Repayments of debt in investments	73.8
Repayments of debt in fixed maturity investments	36.9
Repayments of debt in short-term investments	36.9
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
Bridge Holdings, Ltd. | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Repayments of principal debt			200.0
WM Hinson
Cash flows from financing activities:
Repayments of principal debt	42.4
Proceeds from borrowings	20.0
Repayments of debt in investments	107.6
Repayments of debt in fixed maturity investments	45.9
Repayments of debt in short-term investments	61.7
HG Global
Cash flows from financing activities:
Repayments of principal debt	9.5
Proceeds from borrowings	7.5
White Mountain Investments Luxembourg
Cash flows from financing activities:
Proceeds from borrowings	105.0
White Mountains Adams, Inc.
Cash flows from financing activities:
Distributions, cash		25.0
NSM
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax	0.0	(886.8)	0.0
NSM | White Mountains Insurance Group Ltd.
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax	0.0	(886.8)	0.0
Sirius Group
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax	0.0	0.0	(18.7)
Sirius Group | White Mountains Insurance Group Ltd.
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax	$ 0.0	$ 0.0	$ (18.7)