Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Changes in derivative instruments
Beginning balance			$ 69.2
Realized and unrealized gains (losses)	(26.2)	(63.5)	(32.7)	(245.3)
Ending balance	74.8		74.8
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0.3	1.6	0.3	7.2
Fair Value
Changes in derivative instruments
Beginning balance	89.6	75.8	69.2	98.3
Purchases	0	23.3	0	59.4
Realized and unrealized gains (losses)	(26.2)	(63.5)	(32.7)	(245.3)
Transfers in	0	0	0	0
Sales/settlements	11.4	105.7	38.3	228.9
Ending balance	74.8	141.3	74.8	141.3
Level 2 Inputs
Changes in derivative instruments
Beginning balance	25.1	(42.9)	4.7	(20.5)
Purchases	0	0	0	0
Realized and unrealized gains (losses)	(15.5)	(30.6)	(22.5)	(116.0)
Transfers in	0	0	0	0
Sales/settlements	12.7	91.7	40.1	154.7
Ending balance	22.3	18.2	22.3	18.2
Level 1 Inputs
Changes in derivative instruments
Beginning balance	0.4	(8.7)	1.1	(21.7)
Purchases	0	0	0	0
Realized and unrealized gains (losses)	(0.1)	(8.7)	(0.3)	(55.9)
Transfers in	0	0	0	0
Sales/settlements	(1.1)	14.0	(1.6)	74.2
Ending balance	(0.8)	(3.4)	(0.8)	(3.4)
Variable Annuity | Level 3 Inputs
Changes in derivative instruments
Beginning balance	(54.1)	(251.6)	(52.8)	(441.5)
Realized and unrealized gains (losses)	22.3	57.5	21.0	247.4
Transfers in	0	0	0	0
Sales/settlements	0	0	0	0
Ending balance	(31.8)	(194.1)	(31.8)	(194.1)
Purchases	0		0
Derivative [Member] | Level 3 Inputs
Changes in derivative instruments
Beginning balance	64.1	127.4	63.4	140.5
Purchases	0	23.3	0	59.4
Realized and unrealized gains (losses)	(10.6)	(24.2)	(9.9)	(73.4)
Transfers in	0	0	0	0
Sales/settlements	(0.2)	0	(0.2)	0
Ending balance	$ 53.3	$ 126.5	$ 53.3	$ 126.5