Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended				12 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2011	Jun. 20, 2013
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period					$ 387.9
Equity in earnings	7.1		6.5		16.3		16.7
Carrying value at the end of the period	331.9				331.9
Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	377.5		293.2		381.5		273.6
Equity in earnings	7.2	[1],[2]	7.0	[1],[2]	17.0	[3],[4]	17.9	[3],[4]
Equity in net unrealized losses from Symetra's fixed maturity portfolio	(71.8)	[5]	30.6	[5]	(80.5)	[6]	29.4	[6]
Dividends received	(1.4)		(1.2)		(2.8)		(2.4)
Increase (decrease) in value of warrants	(14.5)		(6.1)		(10.8)		(17.2)
Exercise of warrants	0		0		0		0
Carrying value at the end of the period	326.0	[7],[8],[9]	335.7	[7],[8],[9]	326.0	[7],[8],[9]	335.7	[7],[8],[9]	273.6
Tax on equity in earnings	0.6		0.6		1.4		1.5
Equity in net unrealized losses increase relating to amortization of common share impairment									136.6
Tax on equity in net unrealized gains (losses)	1.4		(2.4)
Aggregate value of White Mountains common shares of Symetra at quoted market price	320.6				320.6
Market Price Per Share	$ 15.99				$ 15.99					$ 15.53
Symetra | Common shareholders' equity.
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	350.9		269.5		351.2		261.0
Equity in earnings	7.2	[1],[2]	7.0	[1],[2]	17.0	[3],[4]	17.9	[3],[4]
Equity in net unrealized losses from Symetra's fixed maturity portfolio	(71.8)	[5]	30.6	[5]	(80.5)	[6]	29.4	[6]
Dividends received	(1.4)		(1.2)		(2.8)		(2.4)
Increase (decrease) in value of warrants	0		0		0		0
Exercise of warrants	41.1		0		41.1		0
Carrying value at the end of the period	326.0	[7],[8],[9]	305.9	[7],[8],[9]	326.0	[7],[8],[9]	305.9	[7],[8],[9]
Impairment of Shares Change in Equity in Earnings	0.8		0.7		1.5		1.8
Equity in net unrealized losses increase relating to amortization of common share impairment	2.8		3.3		5.5		6.6
Symetra | Warrants
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	26.6		23.7		30.3		12.6
Equity in earnings	0	[1],[2]	0	[1],[2]	0	[3],[4]	0	[3],[4]
Equity in net unrealized losses from Symetra's fixed maturity portfolio	0	[5]	0	[5]	0	[6]	0	[6]
Dividends received	0		0		0		0
Increase (decrease) in value of warrants	(14.5)		(6.1)		(10.8)		(17.2)
Exercise of warrants	(41.1)		0		(41.1)		0
Carrying value at the end of the period	0	[7],[8],[9]	29.8	[7],[8],[9]	0	[7],[8],[9]	29.8	[7],[8],[9]
White Mountains
Changes in carrying value of investment in Symetra
Equity in net unrealized losses from Symetra's fixed maturity portfolio	(17.8)		29.4		(17.8)		29.4
Tax on equity in net unrealized gains (losses)					$ 1.4		$ (2.4)

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

[2] Equity in earnings includes $0.8 and $0.7 increase relating to the pre-tax amortization of the Symetra common share impairment.

[3] Equity in earnings excludes tax expense of $1.4 and $1.5.

[4] Equity in earnings includes $1.5 and $1.8 increase relating to the pre-tax amortization of the Symetra common share impairment.

[5] Net unrealized gains includes $2.8 and $3.3 increase relating to the pre-tax amortization of the Symetra common share impairmen

[6] Net unrealized gains includes $5.5 and $6.6 increase relating to the pre-tax amortization of the Symetra common share impairment.

[7] The aggregate value of White Mountains’ investment in common shares of Symetra was $320.6 based upon the quoted market price of $15.99 per share at June 30, 2013.

[8] Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(17.8) and $29.4 as of June 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $1.4 and $(2.4).

[9] Includes White Mountains’ equity in net unrealized gains from Symetra’s fixed maturity portfolio of $(17.8) and $29.4 as of June 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $1.4 and $(2.4).