Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Derivative, Gain (Loss) on Derivative, Net	$ (11.0)	$ 148.9	$ (122.0)	$ 105.4
Derivative, Fair Value, Net	242.1		242.1		290.4
Interest Rate Contract [Member]
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Derivative, Gain (Loss) on Derivative, Net	(22.6)	29.8	(73.1)	17.3
Derivative, Fair Value, Net	29.1		29.1		31.1
Foreign Exchange Contract [Member]
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Derivative, Gain (Loss) on Derivative, Net	6.5	63.0	(21.0)	32.8
Derivative, Fair Value, Net	127.9		127.9		161.3
Equity Contract [Member]
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Derivative, Gain (Loss) on Derivative, Net	5.1	56.1	(27.9)	55.3
Derivative, Fair Value, Net	$ 85.1		$ 85.1		$ 98.0