Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Changes in derivative instruments
Beginning balance	$ 69.2
Realized and unrealized gains (losses)	(6.6)	(181.8)
Ending balance	89.6
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0	(5.6)
Fair Value
Changes in derivative instruments
Purchases	0
Beginning balance	69.2	98.3
Purchases		36.1
Realized and unrealized gains (losses)	6.6	181.8
Transfers in	0	0
Sales/settlements	27.0	123.2
Ending balance	89.6	75.8
Level 3 Inputs
Changes in derivative instruments
Beginning balance	(52.8)	(441.5)
Purchases	0	0
Realized and unrealized gains (losses)	(1.3)	189.9
Transfers in	0	0
Sales/settlements	0	0
Ending balance	(54.1)	(251.6)
Beginning balance	63.4	140.5
Purchases		36.1
Realized and unrealized gains (losses)	0.7	(49.2)
Transfers in	0	0
Sales/settlements	0	0
Ending balance	64.1	127.4
Level 2 Inputs
Changes in derivative instruments
Purchases	0
Beginning balance	4.7	(20.5)
Purchases		0
Realized and unrealized gains (losses)	7.1	85.5
Transfers in	0	0
Sales/settlements	27.5	63.1
Ending balance	25.1	(42.9)
Level 1 Inputs
Changes in derivative instruments
Purchases	0
Beginning balance	1.1	(21.7)
Purchases		0
Realized and unrealized gains (losses)	0.2	47.1
Transfers in	0	0
Sales/settlements	(0.5)	60.1
Ending balance	$ 0.4	$ (8.7)