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SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Operations and Comprehensive Income (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
Payments for (Proceeds from) Short-term Investments									$ 39.0	$ 1.7	$ 27.2
Revenues (loss) (including realized and unrealized gains and losses)	$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8	369.1	373.8	157.7
Expenses	150.1	154.4	134.7	108.1	108.8	79.1	85.7	92.4	547.3	366.0	305.0
Pre-tax loss	(144.1)	44.3	(12.4)	(66.0)	5.2	8.4	(2.2)	(3.6)	(178.2)	7.8	(147.3)
Income tax (expense) benefit	3.6	3.6	(2.5)	(0.7)	2.5	4.0	1.0	0.3	4.0	7.8	32.9
Net income (loss) from continuing operations	(140.5)	47.9	(14.9)	(66.7)	7.7	12.4	(1.2)	(3.3)	(174.2)	15.6	(114.4)
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 loss attributable to non-controlling interests	3.0	10.2	18.4	18.6	10.5	10.6	12.0	1.0	50.2	34.1	(7.2)
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
Other comprehensive (loss) income, net of tax									(4.8)	0.3	(0.7)
Comprehensive (loss) income attributable to White Mountains’s common shareholders									(145.7)	630.5	547.1
Computation of net income (loss) available to common shareholders:
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
Comprehensive income (loss)									(146.0)	630.7	547.4
Total income (loss) from discontinued operations									(17.2)	577.5	523.4
Payments for (Proceeds from) Other Investing Activities									4.2	14.7	(4.8)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									459.2	715.5	1,035.4
Cash Provided by (Used in) Investing Activities, Discontinued Operations									0.0	3.0	241.4
Net Cash Provided by (Used in) Investing Activities									459.2	718.5	1,276.8
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Payments for (Proceeds from) Short-term Investments									(134.0)	24.7	(10.9)
Revenues (loss) (including realized and unrealized gains and losses)									(47.7)	27.3	(1.0)
Expenses									45.9	99.7	68.2
Pre-tax loss									(93.6)	(72.4)	(69.2)
Income tax (expense) benefit									(2.5)	(1.4)	(0.5)
Net income (loss) from continuing operations									(96.1)	(73.8)	(69.7)
Net income from discontinued operations, net of tax									(17.2)	0.0	0.0
Equity in earnings (losses) from consolidated and unconsolidated affiliates									(27.4)	701.0	471.5
Net loss attributable to non-controlling interests									(0.5)	0.0	0.0
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Other comprehensive (loss) income, net of tax									(4.5)	3.3	145.3
Comprehensive (loss) income attributable to White Mountains’s common shareholders									(145.7)	630.5	547.1
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
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
Issuance Of Debt To From Subsidiaries									(55.2)	382.0	992.0
Repayment of debt from subsidiaries									(31.0)	0.0	5.0
Payments to acquire interest in subsidiary									258.2		0.0
Net Cash Provided by (Used in) Investing Activities									498.0	(450.3)	997.9
Guilford Holdings, Inc. | White Mountains Insurance Group Ltd.
Computation of net income (loss) available to common shareholders:
Payments to acquire interest in subsidiary										700.0
Discontinued Operations
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders									(141.2)	627.2	401.8
Total income (loss) from discontinued operations									(17.2)	577.5	523.4
Discontinued Operations | Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)										0.0	253.6
Expenses										0.0	263.1
Net income from discontinued operations, net of tax										0.0	(6.4)
Computation of net income (loss) available to common shareholders:
Comprehensive income (loss)										(0.7)	502.1
Total income (loss) from discontinued operations										(0.7)	356.8
Discontinued Operations | OneBeacon, Sirius Group, and Tranzact
Computation of net income (loss) available to common shareholders:
Total income (loss) from discontinued operations										$ 577.5	$ 523.4
Swedish Tax Authority | Sirius Group | Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income from discontinued operations, net of tax									$ (17.3)

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