Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				3 Months Ended			3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014 Segment, Discontinued Operations	Mar. 31, 2013 Segment, Discontinued Operations	Dec. 31, 2013 Segment, Discontinued Operations	Mar. 31, 2014 Segment, Discontinued Operations OneBeacon Runoff Contract	May 31, 2001 N I C O Cover [Member] Segment, Discontinued Operations OneBeacon Runoff	May 31, 2001 G R C Cover [Member] Segment, Discontinued Operations OneBeacon Runoff	Mar. 31, 2014 GRC and NICO Cover [Member] Segment, Discontinued Operations OneBeacon Runoff	Dec. 31, 2013 GRC and NICO Cover [Member] Segment, Discontinued Operations OneBeacon Runoff
Discontinued Operations [Line Items}
Loss and loss adjustment expense reserves	$ 3,048.4	$ 3,100.6	$ 3,079.3	$ 3,168.9	$ 1,717.3		$ 1,793.1
Net (loss) income from discontinued operations, net of tax	(0.5)	0.5			(0.5)	0.5
Number of contracts seller caused one beacon to purchase prior to acquisition								2
Reinsurance recoverable cover under reinsurance contract									2,500.0	570.0
Adverse loss reserve development on losses occurring in years 2000 and prior										400.0
Loss reserves ceded at acquisition										170.0
Reinsurance recoverables											1,188.8	1,243.7
A M Best Rating System, Number of Debt Ratings								16
Reinsurance recoverable on paid losses								9.5
Reinsurance recoverables, gross of purchase accounting adjustments								1,673.6
Purchase accounting adjustments								$ 133.6