Discontinued Operations Reinsurance (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 OneBeacon Runoff [Member]	Sep. 30, 2012 OneBeacon Contract	Sep. 30, 2012 GRC Cover OneBeacon	Sep. 30, 2012 NICO Cover OneBeacon	May 31, 2001 General Reinsurance Corporation [Member] GRC Cover OneBeacon	Sep. 30, 2012 Segment, Discontinued Operations [Member]	Sep. 30, 2011 Segment, Discontinued Operations [Member]	Sep. 30, 2012 Segment, Discontinued Operations [Member]	Sep. 30, 2011 Segment, Discontinued Operations [Member]	Dec. 31, 2011 Segment, Discontinued Operations [Member]	Sep. 30, 2012 Loss attributable to change in discount rate [Member]	Sep. 30, 2012 Loss attributable to prior year reserve devolopment [Member]
Reinsurance [Line Items]
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent						$ (100.5)					$ (106.8)	$ (30.2)	$ (115.5)	$ (26.0)
Number of Contracts Seller Caused One Beacon to Purchase Prior to Acquisition							2
Reinsurance Recoverable Cover under Reinsurance Contract								570.0	2,500.0
Adverse Loss Reserve Development on Losses Occurring in Years 2000 and Prior										400.0
Loss Reserves Ceded at Acquisition										170.0
Reinsurance Recoverables							1,449.2
Reinsurance recoverable on paid losses	14.8		14.8		30.5		2.0				17.6		17.6		0
Reinsurance Recoverables, Gross of Purchase Accounting Adjustments											2,110.0		2,110.0
Purchase accounting adjustments	153		153
Amount of held for sale to be reinsured at closing	36.7		36.7
Gain (loss) on sale of discontinued operation, net of tax	(91.0)	(18.2)	(91.0)	(18.2)		(91.5)					(91.0)	(18.2)	(91.0)	18.2
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	$ (15.8)	$ (12.0)	$ (24.5)	$ (7.8)							$ (15.8)	$ (12.0)	$ (24.5)	$ (7.8)		$ (9.0)	$ (6.5)