Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended					9 Months Ended
	Sep. 30, 2013		Jun. 30, 2013	Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Changes in variable annuity liabilities
Realized and unrealized gains (losses)			$ 1.5
Changes in derivative instruments
Beginning balance						98.3
Realized and unrealized gains (losses)	63.1			11.0		308.4		122.0
Ending balance	91.5					91.5
Collateral cash and investments	114.8			477.5		114.8		477.5		393.6
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	(1.3)			(0.7)		(8.5)		(6.7)
Fair Value
Changes in derivative instruments
Beginning balance	141.3			250.3		98.3		290.4
Purchases	0			0		59.3		6.1
Realized and unrealized gains (losses)	(63.1)			(11.0)		(308.4)		(122.0)
Transfers in	0			0		0		0
Sales/settlements	13.3			2.8		242.3		67.6
Ending balance	91.5			242.1		91.5		242.1
Level 3 Inputs
Changes in variable annuity liabilities
Beginning balance	(194.1)			(661.8)		(441.5)		(768.5)
Purchases	0			0		0		0
Realized and unrealized gains (losses)	50.4	[1]		(10.6)	[1]	297.8	[1]	96.1	[1]
Transfers in	0			0		0		0
Sales/settlements				0				0
Ending balance	(143.7)			(672.4)		(143.7)		(672.4)
Changes in derivative instruments
Beginning balance	126.5	[2]		213.0	[2]	140.5	[2]	247.1	[2]
Purchases	0	[2]		0	[2]	59.3	[2]	6.1	[2]
Realized and unrealized gains (losses)	(24.1)	[2]		6.7	[2]	(97.4)	[2]	(24.1)	[2]
Transfers in	0	[2]		0	[2]	0	[2]	0	[2]
Sales/settlements		[2]		(6.7)	[2]		[2]	(16.1)	[2]
Ending balance	102.4	[2]		213.0	[2]	102.4	[2]	213.0	[2]
Level 2 Inputs
Changes in derivative instruments
Beginning balance	18.2	[2],[3]		61.0	[2],[3]	(20.5)	[2],[3]	39.2	[2],[3]
Purchases	0	[2],[3]		0	[2],[3]	0	[2],[3]	0	[2],[3]
Realized and unrealized gains (losses)	(28.5)	[2],[3]		(15.3)	[2],[3]	(144.5)	[2],[3]	(71.8)	[2],[3]
Transfers in	0	[2],[3]		0	[2],[3]	0	[2],[3]	0	[2],[3]
Sales/settlements	3.2	[2],[3]		(12.5)	[2],[3]	157.9	[2],[3]	65.8	[2],[3]
Ending balance	(7.1)	[2],[3]		33.2	[2],[3]	(7.1)	[2],[3]	33.2	[2],[3]
Level 1 Inputs
Changes in derivative instruments
Beginning balance	(3.4)	[4]		(23.7)	[4]	(21.7)	[4]	4.1	[4]
Purchases	0	[4]		0	[4]	0	[4]	0	[4]
Realized and unrealized gains (losses)	(10.5)	[4]		(2.4)	[4]	(66.5)	[4]	(26.1)	[4]
Transfers in	0	[4]		0	[4]	0	[4]	0	[4]
Sales/settlements	10.1	[4]		22.0	[4]	84.4	[4]	17.9	[4]
Ending balance	$ (3.8)	[4]		$ (4.1)	[4]	$ (3.8)	[4]	$ (4.1)	[4]

[1] Includes $(1.3) and $(8.5) for the three and nine months ended September 30, 2013 and $(0.7) and $(6.7) for the three and nine months ended September 30, 2012 related to the change in the fair value of variable annuity death benefit liabilities, which are included in general and administrative expenses.

[2] Consists of over-the-counter instruments.

[3] Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index.

[4] Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.