SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders	$ 60.9	$ 48.7	$ 20.5	$ 284.4	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)		$ 414.5	$ (141.2)	$ 627.2
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses										(433.2)	108.3	(133.3)
Net income from discontinued operations, net of tax	0.8	$ (0.9)	$ 0.0	(0.7)	0.0	$ 17.3	$ 0.0	(0.1)		0.0	0.0	(20.5)
Net change in other assets and liabilities, net										18.7	(27.1)	(20.8)
Net cash (used for) provided from operations										(120.5)	(31.1)	94.6
Cash flows from investing activities:
Net decrease in short-term investments										21.9	(39.0)	(1.7)
Net cash (used for) provided from investing activities										(59.9)	459.2	718.5
Cash flows from financing activities:
Draw down of debt and revolving line of credit										206.4	84.1	376.0
Repayment of debt and revolving line of credit										(22.9)	(15.4)	(365.0)
Repurchases and retirement of common shares										0.0	(511.9)	(714.6)
Dividends paid on common shares										(3.2)	(3.8)	(4.6)
Net cash provided from (used for) financing activities										230.5	(414.3)	(699.0)
Net change in cash during the period - continuing operations												(0.9)
Cash balance at beginning of year				110.3				97.1	$ 97.1	110.3	97.1	80.2
Cash balance at end of year	161.0				110.3					161.0	110.3	97.1
Interest paid										(16.3)	(8.8)	(1.4)
White Mountains’s common shareholders’ equity	3,144.7				2,718.2					3,144.7	2,718.2
Amortization of restricted share and option awards										10.5	13.0	14.8
Payable to subsidiary	(6.6)				(4.0)					(6.6)	(4.0)	(11.6)
Lone Tree Holdings Ltd.
Cash flows from financing activities:
White Mountains’s common shareholders’ equity												2,810.4
Other liabilities												14.1
Investments in consolidated subsidiaries												964.4
Short-term investments, at amortized cost												13.0
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders										414.5	(141.2)	627.2
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses										(61.0)	57.8	(18.5)
Undistributed earnings from subsidiaries										(398.5)	27.4	(701.0)
Net income from discontinued operations, net of tax										(0.8)	17.2	0.0
Other non-cash reconciling items, primarily amortization of restricted share and option awards (4)										20.0	34.6	31.1
Accumulated earnings distributed from subsidiary in cash										0.0	0.0	1,256.7
Net change in other assets and liabilities, net										3.0	16.7	(4.9)
Net cash (used for) provided from operations										(22.8)	12.5	1,190.6
Cash flows from investing activities:
Net decrease in short-term investments										(37.6)	134.0	(24.7)
Payments to Acquire Investments										(14.8)	(321.2)	(474.7)
Proceeds from Sale and Maturity of Debt and Equity Securities, FV-NI, Held-for-investment										207.9	967.6	367.1
Issuance of debt (to) from subsidiaries										(83.5)	(55.2)	382.0
Repayment of debt (to) from subsidiaries										5.0	31.0	0.0
Payments to acquire interest in subsidiary										46.1	258.2	700.0
Net cash (used for) provided from investing activities										30.9	498.0	(450.3)
Cash flows from financing activities:
Draw down of debt and revolving line of credit										0.0	0.0	350.0
Repayment of debt and revolving line of credit										0.0	0.0	(350.0)
Repurchases and retirement of common shares										0.0	(510.9)	(714.6)
Dividends paid on common shares										(3.2)	(3.8)	(4.6)
Payments of restricted shares withholding taxes										(4.9)	(10.0)	(9.3)
Net cash provided from (used for) financing activities										(8.1)	(524.7)	(728.5)
Net change in cash during the period - continuing operations										0.0	(14.2)	11.8
Cash balance at beginning of year				$ 0.7				$ 14.9	14.9	0.7	14.9	3.1
Cash balance at end of year	0.7				0.7					0.7	0.7	14.9
Interest paid										0.0	0.0	(0.6)
White Mountains’s common shareholders’ equity	3,261.5				2,843.1					3,261.5	2,843.1
Other liabilities	34.7				25.9					34.7	25.9
Investments in consolidated subsidiaries	2,969.0				2,533.2					2,969.0	2,533.2
Short-term investments, at amortized cost	66.2				28.0					66.2	28.0
Payable to subsidiary	$ 69.3				$ (15.8)					69.3	(15.8)
Transfer from investments												1,238.9
Lone Tree Holdings Ltd. | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Net change in due to affiliates												1,256.7
Proceeds from divestiture of businesses												1,037.6
Bridge Holdings, Ltd. | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Payments to acquire interest in subsidiary											(255.3)
Cash flows from financing activities:
Transfer to investments										70.5	(22.7)
White Mountains Investment, Ltd. | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Payments to acquire interest in subsidiary											(2.9)
Cash flows from financing activities:
Transfer to investments										2.0
Transfer from investments										24.4
HG Global | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer from investments										1.9
Intercompany balances | Lone Tree Holdings Ltd.
Cash flows from financing activities:
Other liabilities												1,863.1
Non-cash reconciling item | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments												$ (94.2)
Non-cash reconciling item | Guilford Holdings, Inc.
Cash flows from financing activities:
Transfer to investments											(349.5)
Noncash Total | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments											603.9
Transfer from investments											1,065.4
Fixed maturity investments | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments											170.5
Transfer from investments											373.4
Fixed maturity investments | Guilford Holdings, Inc.
Cash flows from financing activities:
Transfer from investments											170.4
Common equity securities | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments											148.8
Transfer from investments											490.1
Other long-term investments | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments											22.7
Transfer from investments											22.7
Short-term investments | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Transfer to investments											(284.6)
Transfer from investments											179.2
Short-term investments | Guilford Holdings, Inc.
Cash flows from financing activities:
Transfer from investments											179.2
Bridge Holdings, Ltd. | Guilford Holdings, Inc.
Cash flows from financing activities:
Transfer to investments											(350.0)
Bridge Holdings, Ltd. | White Mountains Investment, Ltd.
Cash flows from financing activities:
Transfer to investments											$ (1.0)
Swedish Tax Authority | Sirius Group | Sirius Group
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net income from discontinued operations, net of tax									$ 17.3	$ (0.8)