SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 69.9	$ 51.3	$ 95.5	$ 95.5	$ 297.6	$ 312.2	$ 321.6
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									(225.4)	(78.5)	(133.9)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(7.3)	$ (14.2)	$ (3.5)	(73.8)	(82.1)	$ (63.9)	$ (61.1)	(51.5)	(80.6)	(260.2)	(99.9)
Net change in other assets and liabilities, net									40.1	(127.5)	(23.0)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									161.0	58.8	(39.5)
Net cash provided from (used for) discontinued operations									14.7	60.0	(61.5)
Net cash provided from (used for) operations									175.7	118.8	(101.0)
Cash flows from investing activities:
Net decrease in short-term investments									140.0	(138.0)	64.6
Purchases of investment securities									(2,377.1)	2,868.5
Sales and maturities of investment securities									2,210.1	(3,500.2)
Receipt of cash flow from discontinued operations									17.5	(88.1)	(72.3)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									(20.0)	12.1	253.1
Net cash provided from investing activities — discontinued operations									31.3	35.8	148.7
Net cash provided from investing activities									11.3	47.9	401.8
Cash flows from financing activities:
Draw down of debt and revolving line of credit									195.5	133.6	200.0
Repayment of debt and revolving line of credit									(95.6)	(66.5)	(275.0)
Repurchases and retirement of common shares									(268.6)	(128.2)	(79.8)
Dividends paid on common shares									(6.0)	(6.2)	(6.2)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(205.5)	(66.4)	(99.0)
Net cash (used for) provided from financing activities — discontinued operations									(9.1)	(63.2)	(87.5)
Net cash used for financing activities									(214.6)	(129.6)	(186.5)
Net change in cash during the period - continuing operations									(64.5)	4.5	114.6
Cash balance at beginning of year				238.0				233.5	238.0	233.5	118.9
Cash balance at end of year	173.5				238.0				173.5	238.0	233.5
Interest Paid									(17.6)	(15.6)	(16.2)
OneBeacon Ltd. common shares repurchased and retired									(3.7)	(1.7)	0.0
Lone Tree Insurance Group Ltd
Cash flows from financing activities:
Cash dividends received on warrants										29.7	28.0
Noncash distribution received											27.9
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders									297.6	312.2	321.6
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									0.0	(0.2)	(0.2)
Undistributed current earnings from subsidiaries									(352.5)	(329.4)	(359.5)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									0.0	(13.9)	0.0
Other non-cash reconciling items including restricted share and option amortization									(0.4)	12.9	15.4
Net change in other assets and liabilities, net									21.4	(7.6)	(2.7)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(33.9)	(26.0)	(25.4)
Net cash provided from (used for) discontinued operations									0.0	13.9	0.0
Net cash provided from (used for) operations									(33.9)	(12.1)	(25.4)
Cash flows from investing activities:
Net decrease in short-term investments									7.6	(29.6)	7.5
Purchases of investment securities									0.0	(7.6)	(26.2)
Sales and maturities of investment securities									0.0	39.4	61.1
Issuance of debt (to) from subsidiaries									271.0	144.6	153.9
Repayment of debt (to) from subsidiaries									(35.0)	(30.0)	(10.3)
Receipt of cash flow from discontinued operations									0.0	13.9	0.0
Payments to subsidiaries									0.0	0.0	0.0
Distributions from subsidiaries									15.0	29.7	0.1
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									258.6	160.4	186.1
Net cash provided from investing activities — discontinued operations									0.0	(13.9)	0.0
Net cash provided from investing activities									258.6	146.5	186.1
Cash flows from financing activities:
Draw down of debt and revolving line of credit									125.0	65.0	200.0
Repayment of debt and revolving line of credit									(75.0)	(65.0)	(275.0)
Proceeds from issuances of common shares									0.0	0.0	0.0
Repurchases and retirement of common shares									(268.6)	(128.2)	(79.8)
Dividends paid on common shares									(6.0)	(6.2)	(6.2)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(224.6)	(134.4)	(161.0)
Net cash (used for) provided from financing activities — discontinued operations									0.0	0.0	0.0
Net cash used for financing activities									(224.6)	(134.0)	(161.0)
Net change in cash during the period - continuing operations									0.1	0.0	(0.3)
Cash balance at beginning of year				$ 0.2				$ 0.2	0.2	0.2	0.5
Cash balance at end of year	$ 0.3				$ 0.2				0.3	0.2	0.2
Interest Paid									$ 0.0	$ (0.3)	(2.0)
Fixed maturity investments | Lone Tree Insurance Group Ltd
Cash flows from financing activities:
Cash dividends received on warrants											27.9
Cash | Lone Tree Insurance Group Ltd
Cash flows from financing activities:
Cash dividends received on warrants											0.1
Revolving Credit Facility [Member] | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Repayment of debt and revolving line of credit											$ (75.0)