Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended		6 Months Ended	3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended					3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 20, 2013 Warrants	Sep. 30, 2013 Symetra		Sep. 30, 2012 Symetra		Sep. 30, 2013 Symetra		Sep. 30, 2012 Symetra		Sep. 30, 2013 Symetra Common shareholders' equity.		Sep. 30, 2012 Symetra Common shareholders' equity.		Sep. 30, 2013 Symetra Common shareholders' equity.		Sep. 30, 2012 Symetra Common shareholders' equity.		Sep. 30, 2013 Symetra Warrants		Sep. 30, 2012 Symetra Warrants		Sep. 30, 2013 Symetra Warrants		Sep. 30, 2012 Symetra Warrants		Jun. 20, 2013 Symetra Warrants	Sep. 30, 2013 White Mountains	Sep. 30, 2012 White Mountains	Sep. 30, 2013 White Mountains	Sep. 30, 2012 White Mountains
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period			$ 387.9			$ 326.0		$ 335.7		$ 381.5		$ 273.6		$ 326.0		$ 305.9		$ 351.2		$ 261.0		$ 0		$ 29.8		$ 30.3		$ 12.6
Equity in earnings	8.6	7.7	24.9	24.4		8.7	[1],[2]	8.8	[1],[2]	25.7	[3],[4]	26.7	[3],[4]	8.7	[1],[2]	8.8	[1],[2]	25.7	[3],[4]	26.7	[3],[4]	0	[1],[2]	0	[1],[2]	0	[3],[4]	0	[3],[4]
Equity in net unrealized losses from Symetra's fixed maturity portfolio						(7.7)	[5]	35.2	[5]	(88.2)	[6]	64.6	[6]	(7.7)	[5]	35.2	[5]	(88.2)	[6]	64.6	[6]	0	[5]	0	[5]	0	[6]	0	[6]		(7.7)	35.2	(25.5)	64.6
Dividends received						(1.8)		(1.3)		(4.6)		(3.7)		(1.8)		(1.3)		(4.6)		(3.7)		0		0		0		0
Increase (decrease) in value of warrants					(14.5)	0		3.6		(10.8)		(13.6)		0		0		0		0		0		3.6		(10.8)		(13.6)
Exercise of warrants						0		0		0		0		0		0		41.1		0		0		0		(41.1)		0
Carrying value at the end of the period	331.7		331.7			325.2	[7],[8],[9]	374.8	[7],[8],[9]	325.2	[7],[8],[9]	374.8	[7],[8],[9]	325.2	[7],[8],[9]	348.6	[7],[8],[9]	325.2	[7],[8],[9]	348.6	[7],[8],[9]	0	[7],[8],[9]	26.2	[7],[8],[9]	0	[7],[8],[9]	26.2	[7],[8],[9]
Tax on equity in earnings						0.6		0.7		2.0		2.2
Impairment of Shares Change in Equity in Earnings														0.7		0.9		2.2		0.9
Equity in net unrealized losses increase relating to amortization of common share impairment														3.1		3.3		8.6		9.9
Tax on equity in net unrealized gains (losses)						0.5		2.8																									2.0	(5.2)
Aggregate value of White Mountains common shares of Symetra at quoted market price						$ 357.3				$ 357.3
Share Price						$ 17.82				$ 17.82																				$ 15.53

[1] Equity in earnings excludes tax expense of $0.6 and $0.7.

[2] Equity in earnings includes $0.7 and $0.9 increase relating to the pre-tax amortization of the Symetra common share basis difference.

[3] Equity in earnings includes $2.2 and $0.9 increase relating to the pre-tax amortization of the Symetra common share basis difference.

[4] Equity in earnings excludes tax expense of $2.0 and $2.2.

[5] Net unrealized gains includes $3.1 and $3.3 increase relating to the pre-tax amortization of the Symetra common share

[6] Net unrealized gains includes $8.6 and $9.9 increase relating to the pre-tax amortization of the Symetra common share basis difference.

[7] Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(25.5) and $64.6 as of September 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $2.0 and $(5.2).

[8] The aggregate value of White Mountains’ investment in common shares of Symetra was $357.3 based upon the quoted market price of $17.82 per share at September 30, 2013.

[9] Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(7.7) and $35.2 as of September 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $0.5 and $2.8.