SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders	$ 792.8	$ (275.4)	$ 708.7
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	93.0	380.3	(686.0)
Net change in other assets and liabilities	90.4	11.7	34.8
Net cash provided from (used for) operations	365.0	38.6	(60.6)
Cash flows from investing activities:
Net change in short-term investments	(455.1)	59.8	60.9
Cash pre-funded for ILS funds managed by Elementum	(70.0)	0.0	0.0
Purchases of consolidated subsidiaries, net of cash acquired	(67.9)	10.6	(9.1)
Net cash provided from (used for) investing activities	115.5	(208.3)	64.6
Cash flows from financing activities:
Repurchases and retirement of common shares	(615.8)	(107.5)	(85.2)
Dividends paid on common shares	(3.0)	(3.1)	(3.2)
Net cash provided from (used for) financing activities – continuing operations	(344.9)	217.6	49.0
Net decrease in cash during the year	107.3	63.1	13.6
Cash balance at beginning of year	147.7	84.6	71.0
Cash balance at end of year	255.0	147.7	84.6
Other long-term investments	1,488.0	1,377.8
Amortization of restricted share awards	14.0	14.7	16.6
Net income (loss) attributable to non-controlling interests	(80.4)	(46.5)	(45.3)
Net change in (receivables) payables to subsidiaries	3.3	6.5	4.8
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders	792.8	(275.4)	708.7
Charges (credits) to reconcile net income to net cash from operations:
Net realized and unrealized investment (gains) losses	5.6	0.1	10.1
Undistributed earnings from consolidated subsidiaries	(853.8)	257.4	(782.0)
Other non-cash reconciling items	(0.4)	14.1	19.0
Accumulated earnings distributed from subsidiary in cash	7.0	0.0	0.0
Net change in other assets and liabilities	17.2	(5.7)	(2.5)
Net cash provided from (used for) operations	(31.6)	(28.2)	(44.4)
Cash flows from investing activities:
Net change in short-term investments	1,165.6	17.7	(127.4)
Purchases of investment securities	(359.2)	0.0	(6.7)
Sales and maturities of investment securities	15.7	0.0	189.7
Purchases of investment securities from subsidiaries	(14.0)	(26.4)	0.0
Sales of investment securities to subsidiaries	0.0	36.4	0.0
Cash pre-funded for ILS funds managed by Elementum	(70.0)	0.0	0.0
Net issuance of debt (to) from subsidiaries	(142.0)	94.0	(44.5)
Net repayment of debt to (from) subsidiaries	(15.0)	0.0	92.6
Purchases of consolidated subsidiaries, net of cash acquired	49.3	17.0	29.1
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.5 $0.0 and $0.0	19.5	0.0	0.0
Net cash provided from (used for) investing activities	649.9	138.7	132.8
Cash flows from financing activities:
Repurchases and retirement of common shares	(615.8)	(107.5)	(85.2)
Dividends paid on common shares	(3.0)	(3.1)	(3.2)
Net cash provided from (used for) financing activities – continuing operations	(618.8)	(110.6)	(88.4)
Net decrease in cash during the year	(0.5)	(0.1)	0.0
Cash balance at beginning of year	0.6	0.7	0.7
Cash balance at end of year	0.1	0.6	0.7
Investments in consolidated subsidiaries	3,026.6	3,661.3
Fixed maturity investments, at fair value	49.2	0.0
Other long-term investments	49.3	0.0
Short-term investments at carrying value	105.2	6.7
Other assets	75.3	3.0
Net income (loss) attributable to non-controlling interests	(7.4)	(2.5)	1.0
Bridge Holdings, Ltd.
Cash flows from investing activities:
Net issuance of debt (to) from subsidiaries	(69.0)
Net repayment of debt to (from) subsidiaries	15.0
Cash flows from financing activities:
Carrying value of stock transferred	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 carrying value	1,358.7
Other assets	6.0
Cash	7.0
Distribution received from subsidiaries		12.6
Bridge Holdings, Ltd. | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Net repayment of debt to (from) subsidiaries		200.0
Purchases of consolidated subsidiaries, net of cash acquired		18.0
Bridge Holdings, Ltd. | White Mountains Insurance Group Ltd. | Short-term investments
Cash flows from investing activities:
Purchases of consolidated subsidiaries, net of cash acquired		17.9
Bridge Holdings, Ltd. | White Mountains Insurance Group Ltd. | Cash
Cash flows from investing activities:
Purchases of consolidated subsidiaries, net of cash acquired		0.1
White Mountains Adams, Inc.
Cash flows from financing activities:
Contribution to subsidiary	25.0
White Mountains Adams, Inc. | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Purchases of consolidated subsidiaries, net of cash acquired		(42.7)
White Mountains Adams, Inc. | White Mountains Insurance Group Ltd. | Short-term investments
Cash flows from investing activities:
Purchases of consolidated subsidiaries, net of cash acquired		(37.1)
White Mountains Adams, Inc. | White Mountains Insurance Group Ltd. | Cash
Cash flows from investing activities:
Purchases of consolidated subsidiaries, net of cash acquired		(5.6)
HG Global
Cash flows from investing activities:
Net issuance of debt (to) from subsidiaries	(6.0)
Cash flows from financing activities:
Distribution received from subsidiaries	116.3	22.3
White Mountains Investments Bermuda
Cash flows from investing activities:
Net issuance of debt (to) from subsidiaries	(205.0)
Cash flows from financing activities:
Contribution to subsidiary	51.0
White Mountains Investments Bermuda | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Purchases of consolidated subsidiaries, net of cash acquired		(19.7)
White Mountains Investments Bermuda | White Mountains Insurance Group Ltd. | Short-term investments
Cash flows from investing activities:
Purchases of consolidated subsidiaries, net of cash acquired		19.1
White Mountains Investments Bermuda | White Mountains Insurance Group Ltd. | Cash
Cash flows from investing activities:
Purchases of consolidated subsidiaries, net of cash acquired		0.6
Group Ark Insurance Limited
Cash flows from financing activities:
Distribution received from subsidiaries	7.7
PSC Holdings, Ltd
Cash flows from financing activities:
Distribution received from subsidiaries	1.3	6.8
Non-cash reconciling item | Bridge Holdings, Ltd.
Cash flows from financing activities:
Transfer to investments			169.6
Short-term investments | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments			169.6
Short-term investments | White Mountains Insurance Group Ltd. | White Mountains Adams, Inc.
Cash flows from financing activities:
Transfer to investments	100.0
WM Lincoln
Cash flows from financing activities:
Carrying value of stock transferred		212.6
Sirius Group
Charges (credits) to reconcile net income to net cash from operations:
Net (gain) loss from sale of discontinued operations, net of tax - Sirius Group	0.0	(18.7)	2.3
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 - Sirius Group	0.0	(18.7)	2.3
Other Operations
Cash flows from investing activities:
Proceeds from the sale of Other Operating Businesses, net of cash sold of $0.5 $0.0 and $0.0	19.5	0.0	0.0
Cash sold in subsidiary transaction	$ 0.5	0.0	0.0
Swedish Tax Authority | Sirius Group
Cash flows from financing activities:
Loss contingency accrual, period increase (decrease)		18.7	(2.3)
Swedish Tax Authority | Sirius Group | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Loss contingency accrual, period increase (decrease)		$ 18.7	$ (2.3)