SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders	$ (275.4)	$ 708.7	$ 414.5
Adjustments to reconcile net income to net cash used for operations:
Net change in other assets and liabilities, net	24.2	30.9	20.4
Net cash provided from (used for) operations	38.6	(60.6)	(120.5)
Cash flows from investing activities:
Net change in short-term investments	54.3	58.7	21.9
Net cash provided from (used for) investing activities	(208.3)	64.6	(59.9)
Cash flows from financing activities:
Repurchases and retirement of common shares	(100.1)	(78.5)	0.0
Dividends paid on common shares	(3.1)	(3.2)	(3.2)
Net cash provided from (used for) financing activities	217.6	49.0	230.5
Net decrease in cash during the year	48.1	50.2	50.7
Cash balance at beginning of year	211.2	161.0	110.3
Cash balance at end of year	259.3	211.2	161.0
Amortization of restricted share awards	14.7	16.6	10.5
Net change in (receivables) payables to subsidiaries	6.5	4.8	6.6
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders	(275.4)	708.7	414.5
Adjustments to reconcile net income to net cash used for operations:
Net realized and unrealized investment (gains) losses from MediaAlpha	0.1	10.1	(61.0)
Undistributed earnings from subsidiaries	257.4	(782.0)	(398.5)
Net (gain) loss from sale of discontinued operations, net of tax	(18.7)	2.3	(0.8)
Other non-cash reconciling items, primarily amortization of restricted shares and option awards	14.1	19.0	20.0
Net change in other assets and liabilities, net	(5.7)	(2.6)	3.0
Net cash provided from (used for) operations	(28.2)	(44.5)	(22.8)
Cash flows from investing activities:
Net change in short-term investments	17.7	(127.4)	(37.6)
Purchases of investment securities	0.0	(6.7)	(14.8)
Sales and maturities of investment securities	0.0	189.7	207.9
Purchases of investment securities from subsidiaries	(26.4)	0.0	0.0
Sales of investment securities to subsidiaries	36.4	0.0	0.0
Issuance of debt (to) from subsidiaries	94.0	(44.5)	(83.5)
Repayment of debt to (from) subsidiaries	0.0	92.6	5.0
Net distributions from (contributions to) subsidiaries	17.0	29.1	(46.1)
Net cash provided from (used for) investing activities	138.7	132.8	30.9
Cash flows from financing activities:
Repurchases and retirement of common shares	(100.1)	(78.5)	0.0
Dividends paid on common shares	(3.1)	(3.2)	(3.2)
Payments of restricted shares withholding taxes	(7.4)	(6.6)	(4.9)
Net cash provided from (used for) financing activities	(110.6)	(88.3)	(8.1)
Net decrease in cash during the year	(0.1)	0.0	0.0
Cash balance at beginning of year	0.7	0.7	0.7
Cash balance at end of year	0.6	0.7	0.7
Bridge Holdings, Ltd.
Cash flows from financing activities:
Excess of intercompany debt	12.6
Bridge Holdings, Ltd. | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Repayment of debt to (from) subsidiaries	200.0
Net distributions from (contributions to) subsidiaries	18.0
Cash flows from financing activities:
Proceeds from Contributions from Parent			(70.5)
Bridge Holdings, Ltd. | White Mountains Insurance Group Ltd. | Short-term investments
Cash flows from investing activities:
Net distributions from (contributions to) subsidiaries	17.9
Bridge Holdings, Ltd. | White Mountains Insurance Group Ltd. | Cash
Cash flows from investing activities:
Net distributions from (contributions to) subsidiaries	0.1
HG Global | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Proceeds from Contributions from Affiliates		22.3	1.9
White Mountains Adams, Inc. | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Net distributions from (contributions to) subsidiaries	(42.7)
White Mountains Adams, Inc. | White Mountains Insurance Group Ltd. | Short-term investments
Cash flows from investing activities:
Net distributions from (contributions to) subsidiaries	(37.1)
White Mountains Adams, Inc. | White Mountains Insurance Group Ltd. | Cash
Cash flows from investing activities:
Net distributions from (contributions to) subsidiaries	(5.6)
White Mountains Investments Bermuda | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Proceeds from Contributions from Parent			(2.0)
Proceeds from Contributions from Affiliates			24.4
PSC Holdings, Ltd | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Proceeds from Contributions from Affiliates		6.8
Non-cash reconciling item | Guilford Holdings, Inc.
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
WM Lincoln
Cash flows from financing activities:
Carrying value of stock transferred	212.6
White Mountains Investments Bermuda | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Proceeds from Contributions from Affiliates	(19.7)
White Mountains Investments Bermuda | White Mountains Insurance Group Ltd. | Short-term investments
Cash flows from financing activities:
Proceeds from Contributions from Affiliates	(19.1)
White Mountains Investments Bermuda | White Mountains Insurance Group Ltd. | Cash
Cash flows from financing activities:
Proceeds from Contributions from Affiliates	(0.6)
Swedish Tax Authority | Sirius Group
Cash flows from financing activities:
Loss contingency accrual, period increase (decrease)	18.7	(2.3)	0.8
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)	$ 0.8