Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended																		3 Months Ended	6 Months Ended	12 Months Ended							12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Symetra		Dec. 31, 2012 Symetra		Dec. 31, 2011 Symetra		Dec. 31, 2013 Common Stock Symetra		Dec. 31, 2012 Common Stock Symetra		Dec. 31, 2011 Common Stock Symetra		Dec. 31, 2013 Common Stock White Mountains	Dec. 31, 2012 Common Stock White Mountains	Dec. 31, 2011 Common Stock White Mountains	Jun. 30, 2013 Warrants	Jun. 20, 2013 Warrants	Dec. 31, 2013 Warrants Symetra		Dec. 31, 2012 Warrants Symetra		Dec. 31, 2011 Warrants Symetra		Jun. 20, 2013 Warrants Symetra	Dec. 31, 2013 equity in earnings of unconsolidated affiliates [Member] Common Stock Symetra	Dec. 31, 2012 equity in earnings of unconsolidated affiliates [Member] Common Stock Symetra	Dec. 31, 2011 equity in earnings of unconsolidated affiliates [Member] Common Stock Symetra		Dec. 31, 2013 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra	Dec. 31, 2012 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra	Dec. 31, 2011 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra		Dec. 31, 2013 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock White Mountains	Dec. 31, 2012 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock White Mountains	Dec. 31, 2011 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock White Mountains	Dec. 31, 2011 impairment of equity in earnings [Member] equity in earnings of unconsolidated affiliates [Member] Common Stock Symetra	Dec. 31, 2013 impairment of net unrealized gains (losses) [Member] Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra	Dec. 31, 2012 impairment of net unrealized gains (losses) [Member] Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra	Dec. 31, 2011 impairment of net unrealized gains (losses) [Member] Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra	Dec. 31, 2013 Amortization of common share basis difference [Member] Common Stock Symetra	Dec. 31, 2012 Amortization of common share basis difference [Member] Common Stock Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period				$ 387.9					$ 387.9			$ 381.5	[1]	$ 273.6	[1]	$ 387.5		$ 351.2	[1]	$ 261.0	[1]	$ 350.4							$ 30.3	[1]	$ 12.6	[1]	$ 37.1
Equity in earnings (losses) of unconsolidated affiliates, net of tax	11.7	8.6	7.1	9.2	5.5	7.7	6.5	10.2	36.6	29.9	(20.2)	37.8		32.3	[2],[3],[4]	28.2	[2],[3]	37.8		32.3	[2],[3],[4]	28.2	[2],[3]						0		0	[2],[3],[4]	0	[2],[3]																	3.0	3.5
Impairment of equity in earnings of Symetra																																	0	[5]				(50.0)	[6]			(148.6)	[5]
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates									(106.4)	62.8	(63.6)	(106.4)	[7],[8]	62.8	[7],[8]	85.0	[7]	(106.4)	[7],[8]	62.8	[7],[8]	85.0	[7]	(43.6)	62.8	0			0	[7],[8]	0	[7],[8]	0	[7]														11.8	13.1
Dividends received												(6.4)		(4.9)		(4.0)		(6.4)		(4.9)		(4.0)							0		0		0
Increase (decrease) in value of warrants												10.8		17.7		(24.5)		0		0		0					14.5	10.8	10.8		17.7		(24.5)
Carrying value at the end of the period	321.4				387.9				321.4	387.9		317.3	[2],[9]	381.5	[1]	273.6	[1]	317.3	[2],[9]	351.2	[1]	261.0	[1]						0	[2],[9]	30.3	[1]	12.6	[1]
Unrealized (losses) gains from Symetra’s fixed maturity portfolio												(43.6)		62.8
Income Tax Expense (Benefit)	27.4	8.2	(0.6)	41.6	(101.0)	47.8	6.4	31.1	76.6	(15.7)	(110.0)																									2.8	2.6	2.3		(8.3)	5.1	6.9		(3.2)	5.1	0	(4.1)			(12.0)
Loss from dilutive effect resulting from dividend and issuance of restricted shares																		0.2		1.3		1.0
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																		380.1
Share Price																		$ 18.96																	$ 15.53
Exercise of warrants												$ 0						$ 41.1											$ (41.1)

[1] Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(43.6), $62.8, and $0 as of December 31, 2013, 2012 and 2011, which excludes tax benefit (expense) of $3.2, $(5.1) and $0

[2] Equity in earnings for the years end December 31, 2013, 2012 and 2011 excludes tax expense of $2.8, $2.6, and $2.3

[3] Equity in earnings for the years end December 31, 2013, 2012 and 2011 includes $.2, $1.3, and $1.0 loss from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra

[4] Equity in earnings for the years ended December 31, 2013 and 2012 includes $3.0 and $3.5 increases relating to the pre-tax amortization of the Symetra common share basis difference.

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

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

[7] Net unrealized (losses) gains from Symetra’s fixed maturity portfolio excludes tax benefit (expense) of $8.3, $(5.1) and $(6.9) for the years ended December 31, 2013, 2012 and 2011.

[8] Net unrealized gains for the years ended December 31, 2013 and 2012 includes $11.8 and $13.1 increases relating to the pre-tax amortization of the Symetra common share basis difference.

[9] The aggregate value of White Mountains’ investment in common shares of Symetra was $380.1 based upon the quoted market price of $18.96 per share at December 31, 2013.