SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains’s common shareholders	$ (33,300,000)	$ 91,700,000	$ 341,100,000	$ 13,000,000	$ 267,900,000	$ (58,900,000)	$ 4,300,000	$ 84,300,000	$ 412,500,000	$ 297,600,000	$ 312,200,000
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									(10,300,000)	(225,400,000)	(78,500,000)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	6,800,000	$ (61,900,000)	$ (358,600,000)	(1,100,000)	(19,600,000)	$ (6,500,000)	$ 500,000	(71,300,000)	300,000	(78,700,000)	(260,600,000)
Net change in other assets and liabilities, net									(226,700,000)	49,700,000	(142,400,000)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(130,600,000)	119,700,000	69,900,000
Net cash (used for) provided from discontinued operations									(24,700,000)	56,000,000	48,900,000
Net cash (used for) provided from operations									(155,300,000)	175,700,000	118,800,000
Cash flows from investing activities:
Net decrease in short-term investments									(70,100,000)	140,000,000	(138,000,000)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									1,055,100,000	16,700,000	(3,300,000)
Net cash provided from investing activities — discontinued operations									221,700,000	(5,400,000)	51,200,000
Net cash provided from investing activities									1,276,800,000	11,300,000	47,900,000
Cash flows from financing activities:
Draw down of debt and revolving line of credit									352,500,000	171,500,000	133,600,000
Repayment of debt and revolving line of credit									(404,600,000)	(76,100,000)	(65,200,000)
Proceeds from issuances of common shares									3,700,000	0	0
Repurchases and retirement of common shares									(881,300,000)	(268,600,000)	(128,200,000)
Dividends paid on common shares									(5,400,000)	(6,000,000)	(6,200,000)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(942,200,000)	(203,000,000)	(65,100,000)
Net cash used for financing activities — discontinued operations									(200,000)	(11,600,000)	(64,500,000)
Net cash used for financing activities									(942,400,000)	(214,600,000)	(129,600,000)
Net change in cash during the period - continuing operations									(17,700,000)	(66,600,000)	1,500,000
Cash balance at beginning of year				167,200,000				232,700,000	167,200,000	232,700,000	233,500,000
Cash balance at end of year	149,800,000				167,200,000				149,800,000	167,200,000	232,700,000
Interest Paid									(14,800,000)	(12,800,000)	(13,000,000)
OneBeacon Ltd. common shares repurchased and retired									(11,500,000)	(3,700,000)	(1,700,000)
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Payable to subsidiary (2)									200,000.0	2,400,000.0	700,000.0
Transfer from Investments									80,000,000.0
Net income attributable to White Mountains’s common shareholders									412,500,000	297,600,000	312,200,000
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment gains									1,100,000	0	(200,000)
Undistributed current earnings from subsidiaries									(482,200,000)	(352,500,000)	(329,400,000)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									0	0	(13,900,000)
Other non-cash reconciling items including restricted share and option amortization									17,900,000	(400,000)	12,900,000
Net change in other assets and liabilities, net									(11,400,000)	21,400,000	(7,600,000)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(62,100,000)	(33,900,000)	(26,000,000)
Net cash (used for) provided from discontinued operations									0	0	13,900,000
Net cash (used for) provided from operations									(62,100,000)	(33,900,000)	(12,100,000)
Cash flows from investing activities:
Net decrease in short-term investments									10,900,000	7,600,000	(29,600,000)
Purchases of investment securities									0	0	(7,600,000)
Sales and maturities of investment securities									0	0	39,400,000
Issuance of debt (to) from subsidiaries									992,000,000	271,000,000	144,600,000
Repayment of debt (to) from subsidiaries									(5,000,000)	(35,000,000)	(30,000,000)
Receipt of cash flow from discontinued operations									0	0	13,900,000
Distributions from subsidiaries									0	15,000,000	29,700,000
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									997,900,000	258,600,000	160,400,000
Net cash provided from investing activities — discontinued operations									0	0	(13,900,000)
Net cash provided from investing activities									997,900,000	258,600,000	146,500,000
Cash flows from financing activities:
Draw down of debt and revolving line of credit									350,000,000	125,000,000	65,000,000
Repayment of debt and revolving line of credit									(400,000,000)	(75,000,000)	(65,000,000)
Proceeds from issuances of common shares									3,700,000	0	0
Repurchases and retirement of common shares									(881,300,000)	(268,600,000)	(128,200,000)
Dividends paid on common shares									(5,400,000)	(6,000,000)	(6,200,000)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(933,000,000)	(224,600,000)	(134,400,000)
Net cash used for financing activities — discontinued operations									0	0	0
Net cash used for financing activities									(933,000,000)	(225,000,000)	(134,400,000)
Net change in cash during the period - continuing operations									2,800,000	100,000	0
Cash balance at beginning of year				$ 300,000				$ 200,000	300,000	200,000	200,000
Cash balance at end of year	$ 3,100,000				$ 300,000				3,100,000	300,000	200,000
Proceeds from Dividends										15,000,000	29,700,000
Interest Paid									$ (1,200,000)	$ 0	$ (300,000)