Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period					$ 275.3								$ 275.3
Equity in earnings (losses) of unconsolidated affiliates	5.5		7.7	6.5	10.2		(36.4)		1.5	7.9	6.8		29.9		(20.2)		9.9
Impairment of equity in earnings of Symetra															0	[1]
Impairment of net unrealized gains from Symetra's fixed maturity portolio															0	[2]
Carrying value at the end of the period	387.9						275.3						387.9		275.3
Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period					273.6	[3]					387.5	[3]	273.6	[3]	387.5	[3]	307.7	[3]
Equity in earnings (losses) of unconsolidated affiliates													32.3	[4],[5],[6]	28.2	[4],[5]	11.1	[4],[5],[7]
Impairment of equity in earnings of Symetra															(50.0)	[1]
Equity in net unrealized losses from Symetra's fixed maturity portfolio													62.8	[8]	85.0	[8]	72.7	[8],[9]
Impairment of net unrealized gains from Symetra's fixed maturity portolio															(148.6)	[2]
Dividends received													(4.9)		(4.0)		(2.6)
Increase (decrease) in value of warrants													17.7		(24.5)		(1.4)
Carrying value at the end of the period	381.5	[10],[3]					273.6	[3]					381.5	[10],[3]	273.6	[3]	387.5	[3]
Tax on equity in earnings													2.6		2.3		1.4
Unrealized gains from Symetra’s fixed maturity portfolio													62.8		0		63.7
Tax on equity in net unrealized gains (losses)													5.1		0		5.2
Loss from silutive effect resulting from public offering																	17.9
Tax Benefit on other than temporary impairment															4.1
Impairment of net unrealized gains															12.0
Gain from dilutive effect restulitng from public offering																	1.9
Net unrealized gains (losses) from Symetra's fixed maturity portfolio													5.1		6.9		(0.8)
Loss from dilutive effect resulting from dividend and issuance of restricted shares													1.3		1.0		0.8
Equity in net unrealized losses increase relating to amortization of common share impairment															136.6
Aggregate value of White Mountains common shares of Symetra at quoted market price	225.9												225.9
Quoted market price (in dollars per share)	$ 12.98												$ 12.98
Symetra | Common shareholders’ equity
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period					261.0	[3]					350.4	[3]	261.0	[3]	350.4	[3]	269.2	[3]
Equity in earnings (losses) of unconsolidated affiliates													32.3	[4],[5],[6]	28.2	[4],[5]	11.1	[4],[5],[7]
Impairment of equity in earnings of Symetra															(50.0)	[1]
Equity in net unrealized losses from Symetra's fixed maturity portfolio													62.8	[8]	85.0	[8]	72.7	[8],[9]
Impairment of net unrealized gains from Symetra's fixed maturity portolio															(148.6)	[2]
Dividends received													(4.9)		(4.0)		(2.6)
Increase (decrease) in value of warrants													0		0		0
Carrying value at the end of the period	351.2	[10],[3]					261.0	[3]					351.2	[10],[3]	261.0	[3]	350.4	[3]
Equity in earnings increase relating to amortization of common share impairment													3.5
Equity in net unrealized losses increase relating to amortization of common share impairment													13.1
Symetra | Warrants
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period					12.6	[3]					37.1	[3]	12.6	[3]	37.1	[3]	38.5	[3]
Equity in earnings (losses) of unconsolidated affiliates													0	[4],[5],[6]	0	[4],[5]	0	[4],[5],[7]
Equity in net unrealized losses from Symetra's fixed maturity portfolio													0	[8]	0	[8]	0	[8],[9]
Dividends received													0		0		0
Increase (decrease) in value of warrants													17.7		(24.5)		(1.4)
Carrying value at the end of the period	$ 30.3	[10],[3]					$ 12.6	[3]					$ 30.3	[10],[3]	$ 12.6	[3]	$ 37.1	[3]

[1] Impairment of equity in earnings of Symetra excludes tax benefit of $4.1

[2] Impairment of net unrealized gains from Symetra’s fixed maturity portfolio excludes tax benefit of $12.0

[3] Includes White Mountains’ equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio of $62.8, $0, and $63.7 as of December 31, 2012, 2011 and 2010, which excludes tax expense of $5.1, $0 and $5.2

[4] Equity in earnings for the years end December 31, 2012, 2011 and 2010 includes $1.3, $1.0, and $0.8 loss from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra

[5] Equity in earnings for the years end December 31, 2012, 2011 and 2010 excludes tax expense of $2.6, $2.3, and $1.4

[6] Equity in earnings includes $3.5 increase relating to the pre-tax amortization of Symetra common share impairment from December 31, 2012.

[7] Includes a $17.9 loss from the dilutive effect of Symetra’s public offering

[8] Net unrealized gains (losses) from Symetra’s fixed maturity portfolio excludes tax (expense) benefit of $(5.1), $(6.9) and $0.8 for the years ended December 31, 2012, 2011 and 2010.

[9] Includes a $1.9 gain from the dilutive effect of Symetra’s public offering.

[10] The aggregate value of White Mountains’ investment in common shares of Symetra was $225.9 based upon the quoted market price of $12.98 per share at December 31, 2012.