SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended			1 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2011
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders	$ 70.0	$ 51.5	$ 95.6	$ 95.6	$ 117.9	$ 57.2	$ 26.3	$ 120.4	$ 312.7	$ 321.8	$ 207.4
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									(283.9)	(161.7)	(118.2)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	12.2	(6.7)	(2.6)	0.5	0.3	(0.4)	(3.9)	(0.5)	1.8	42.1	24.0
Net change in other assets and liabilities, net									57.1	2.6	(103.5)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									206.9	(28.7)	(29.6)
Net cash used for discontinued operations									(88.1)	(72.3)	(196.2)
Net cash provided from (used for) operations									118.8	(101.0)	(225.8)
Cash flows from investing activities:
Net decrease in short-term investments									(351.5)	34.8	145.3
Purchases of investment securities									(5,602.1)	(4,635.5)
Sales and maturities of investment securities									6,387.5	4,979.8
Receipt of cash flow from discontinued operations									(88.1)	(72.3)	(196.2)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									(40.2)	329.5	633.4
Net cash provided from investing activities — discontinued operations									88.1	72.3	196.2
Net cash provided from investing activities									47.9	401.8	829.6
Cash flows from financing activities:
Draw down of revolving line of credit									0	200.0	150.0
Repayment of revolving line of credit									(66.5)	(275.0)	(75.0)
Repurchases and retirement of common shares									(128.2)	(79.8)	(669.1)
Dividends paid on common shares									(6.2)	(6.2)	(6.6)
Net cash used for financing activities									(129.6)	(186.5)	(648.2)
Net change in cash during the period									22.8	114.1	(41.3)
Cash balance at beginning of year	349.5				326.7				349.5	326.7	212.6	326.7	251.9
Cash balance at end of year	349.5				326.7				349.5	326.7	212.6	326.7	251.9
Interest Paid									(42.0)	(42.6)	(44.6)
Common shares repurchased and retired during the period										1,329,640
OneBeacon Ltd. common shares repurchased and retired									(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									312.7	321.8	207.4
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									(0.2)	(0.2)	(11.0)
Undistributed current earnings from subsidiaries									(329.9)	(359.7)	(219.3)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									(13.9)	0	0
Other non-cash reconciling items including restricted share and option amortization									12.9	15.6	22.1
Net change in other assets and liabilities, net									(7.6)	(2.9)	(10.0)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(26.0)	(25.4)	(10.8)
Net cash used for discontinued operations									13.9	0	0
Net cash provided from (used for) operations									(12.1)	(25.4)	(10.8)
Cash flows from investing activities:
Net decrease in short-term investments									(29.6)	7.5	63.3
Purchases of investment securities									(7.6)	(26.2)	(706.2)
Sales and maturities of investment securities									39.4	61.1	2,009.7
Issuance of debt (to) from subsidiaries									144.6	153.9	(121.0)
Repayment of debt (to) from subsidiaries									(30.0)	(10.3)	28.5
Receipt of cash flow from discontinued operations									13.9	0	0
Payments to subsidiaries									0	0	(663.0)
Distributions from subsidiaries									29.7	0.1	0
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									160.4	186.1	611.3
Net cash provided from investing activities — discontinued operations									(13.9)	0	0
Net cash provided from investing activities									146.5	186.1	611.3
Cash flows from financing activities:
Draw down of revolving line of credit									65.0	200.0	150.0
Repayment of revolving line of credit									(65.0)	(275.0)	(75.0)
Proceeds from issuances of common shares									0	0	0
Repurchases and retirement of common shares									(128.2)	(79.8)	(669.1)
Dividends paid on common shares									(6.2)	(6.2)	(6.6)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(134.4)	(161.0)	(600.7)
Cash Provided by (Used in) Financing Activities, Discontinued Operations									0	0	0
Net cash used for financing activities									(134.4)	(161.0)	(600.7)
Net change in cash during the period									0	(0.3)	(0.2)
Cash balance at beginning of year	0.2				0.2				0.2	0.2	0.5	0.2	0.7
Cash balance at end of year	0.2				0.2				0.2	0.2	0.5	0.2	0.7
Interest Paid									(0.3)	(2.0)	(12.4)
OneBeacon Ltd. common shares repurchased and retired											(663.0)
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 revolving line of credit										$ (75.0)