SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (141.2)	$ 627.2	$ 401.8
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment losses (gains)									108.3	(133.3)	27.4
Net income from discontinued operations, net of tax	0.0	$ (17.3)	$ 0.0	0.1	4.3	$ 539.1	$ 2.8	31.3	0.0	20.5	108.3
Net change in other assets and liabilities, net									(27.1)	(20.8)	(146.1)
Net cash (used for) provided from operations									(31.1)	94.6	(155.1)
Cash flows from investing activities:
Net decrease in short-term investments									(39.0)	(1.7)	(27.2)
Net cash provided from investing activities									459.2	718.5	1,276.8
Cash flows from financing activities:
Draw down of debt and revolving line of credit									84.1	376.0	352.5
Repayment of debt and revolving line of credit									(15.4)	(365.0)	(404.6)
Proceeds from issuances of common shares									0.0	0.0	3.7
Repurchases and retirement of common shares									(511.9)	(714.6)	(881.3)
Dividends paid on common shares									(3.8)	(4.6)	(5.4)
Net cash used for financing activities									(414.3)	(699.0)	(948.4)
Net change in cash during the period - continuing operations									13.2	16.0	2.1
Cash balance at beginning of year				97.1				80.2	97.1	80.2	77.8
Cash balance at end of year	110.3				97.1				110.3	97.1	80.2
Interest paid									(8.8)	(1.4)	(2.1)
White Mountains’s common shareholders’ equity	2,718.2				3,360.8				2,718.2	3,360.8
Amortization of restricted share and option awards									13.0	14.8	18.5
Payable to subsidiary	4.0				11.6				4.0	11.6	0.2
Lone Tree Holdings Ltd.
Cash flows from financing activities:
White Mountains’s common shareholders’ equity					2,810.4					2,810.4
Other liabilities					14.1					14.1
Investments in consolidated subsidiaries					964.4					964.4
Short-term investments, at amortized cost					13.0					13.0
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment losses (gains)									57.8	(18.5)	1.1
Undistributed earnings from subsidiaries									27.4	(701.0)	(471.5)
Net income from discontinued operations, net of tax									(17.2)	0.0	0.0
Other non-cash reconciling items, primarily amortization of restricted share and option awards (3)									34.6	31.1	17.9
Accumulated earnings distributed from subsidiary in cash									0.0	1,256.7	0.0
Net change in other assets and liabilities, net									16.7	(4.9)	(5.6)
Net cash (used for) provided from operations									12.5	1,190.6	(56.3)
Cash flows from investing activities:
Net decrease in short-term investments									134.0	(24.7)	10.9
Payments to Acquire Investments									(321.2)	(474.7)	0.0
Proceeds from Sale and Maturity of Debt and Equity Securities, FV-NI, Held-for-investment									967.6	367.1	0.0
Repayment of debt (to) from subsidiaries									31.0	0.0	(5.0)
Payments to acquire interest in subsidiary									258.2		0.0
Net cash provided from investing activities									498.0	(450.3)	997.9
Cash flows from financing activities:
Draw down of debt and revolving line of credit									0.0	350.0	350.0
Repayment of debt and revolving line of credit									0.0	(350.0)	(400.0)
Proceeds from issuances of common shares									0.0	0.0	3.7
Repurchases and retirement of common shares									(510.9)	(714.6)	(881.3)
Dividends paid on common shares									(3.8)	(4.6)	(5.4)
Payments of restricted shares withholding taxes									(10.0)	(9.3)	(5.8)
Net cash used for financing activities									(524.7)	(728.5)	(938.8)
Net change in cash during the period - continuing operations									(14.2)	11.8	2.8
Cash balance at beginning of year				$ 14.9				$ 3.1	14.9	3.1	0.3
Cash balance at end of year	0.7				14.9				0.7	14.9	3.1
Interest paid									0.0	(0.6)	(1.2)
White Mountains’s common shareholders’ equity	2,843.1				3,492.5				2,843.1	3,492.5
Other liabilities	25.9				21.2				25.9	21.2
Investments in consolidated subsidiaries	2,533.2				1,914.8				2,533.2	1,914.8
Short-term investments, at amortized cost	28.0				57.2				28.0	57.2
Payable to subsidiary	15.8				11.8				15.8	11.8
Transfer from investments										1,238.9
Sirius Group
Cash flows from financing activities:
Other liabilities	$ 17.3								17.3
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:
Noncash Investing and Financing Activities Related Text									22.7
White Mountains Investment, Ltd. | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Payments to acquire interest in subsidiary									$ (2.9)
Guilford Holdings, Inc. | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Payments to acquire interest in subsidiary										700.0
Intercompany balances | Lone Tree Holdings Ltd.
Cash flows from financing activities:
Other liabilities					$ 1,863.1					$ 1,863.1
Non-cash reconciling item | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text										94.2
Non-cash reconciling item | Guilford Holdings, Inc.
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text									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		80.0
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. | White Mountains Investment, Ltd.
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text									1.0
Bridge Holdings, Ltd. | Guilford Holdings, Inc.
Cash flows from financing activities:
Noncash Investing and Financing Activities Related Text									350.0
Sirius Group
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment losses (gains)											$ (3.7)
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)