Significant Transactions (OneBeacon Runoff Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Significant Transactions
Net loss from other discontinued operations	$ (0.5)	$ 106.8	$ (0.5)	$ 9.2	$ (647.6)	$ 30.1	$ 2.4	$ (6.5)	$ 24.0	$ 36.7	$ 30.1
OneBeacon Runoff
Significant Transactions
Gain (loss) on sale of discontinued operation, net of tax									(91.5)
Loss attributable to change in discount rate | OneBeacon Runoff
Significant Transactions
Net loss from other discontinued operations									$ 9.0