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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Changes in derivative instruments
Beginning balance			$ 69.2
Realized and unrealized gains (losses)	(23.8)	(63.1)	(56.5)	(308.4)
Ending balance	65.3		65.3
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0.2	1.3	0.6	8.5
Fair Value
Changes in derivative instruments
Beginning balance	74.8	141.3	69.2	98.3
Purchases	0	0	0	59.3
Realized and unrealized gains (losses)	(23.8)	(63.1)	(56.5)	(308.4)
Transfers in	0	0	0	0
Sales/settlements	14.3	13.3	52.6	242.3
Ending balance	65.3	91.5	65.3	91.5
Level 2 Inputs
Changes in derivative instruments
Beginning balance	22.3	18.2	4.7	(20.5)
Purchases	0	0	0	0
Realized and unrealized gains (losses)	(19.8)	(28.5)	(42.1)	(144.5)
Transfers in	0	0	0	0
Sales/settlements	22.5	3.2	62.4	157.9
Ending balance	25.0	(7.1)	25.0	(7.1)
Level 1 Inputs
Changes in derivative instruments
Beginning balance	(0.8)	(3.4)	1.1	(21.7)
Purchases	0	0	0	0
Realized and unrealized gains (losses)	13.4	(10.5)	13.1	(66.5)
Transfers in	0	0	0	0
Sales/settlements	(8.2)	10.1	(9.8)	84.4
Ending balance	4.4	(3.8)	4.4	(3.8)
Variable Annuity | Level 3 Inputs
Changes in derivative instruments
Beginning balance	(31.8)	(194.1)	(52.8)	(441.5)
Realized and unrealized gains (losses)	20.1	50.4	41.1	297.8
Transfers in	0	0	0	0
Sales/settlements	0	0	0	0
Ending balance	(11.7)	(143.7)	(11.7)	(143.7)
Purchases	0		0
Derivative [Member] | Level 3 Inputs
Changes in derivative instruments
Beginning balance	53.3	126.5	63.4	140.5
Purchases	0	0	0	59.3
Realized and unrealized gains (losses)	(17.4)	(24.1)	(27.5)	(97.4)
Transfers in	0	0	0	0
Sales/settlements	0	0	0	0
Ending balance	$ 35.9	$ 102.4	$ 35.9	$ 102.4