Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended					3 Months Ended			3 Months Ended			3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Segment, Discontinued Operations	Mar. 31, 2012 Segment, Discontinued Operations	Dec. 31, 2012 Segment, Discontinued Operations	Mar. 31, 2013 Segment, Discontinued Operations OneBeacon Runoff Contract	Dec. 31, 2012 Segment, Discontinued Operations OneBeacon Runoff	Mar. 31, 2013 Segment, Discontinued Operations GRC Cover OneBeacon Runoff	Mar. 31, 2013 Segment, Discontinued Operations NICO Cover OneBeacon Runoff	May 31, 2001 Segment, Discontinued Operations General Reinsurance Corporation GRC Cover OneBeacon Runoff	Mar. 31, 2013 OneBeacon Runoff Segment, Discontinued Operations	Mar. 31, 2012 OneBeacon Runoff Segment, Discontinued Operations	Feb. 22, 2012 AutoOne Entity	Mar. 31, 2013 OneBeacon Runoff Segment, Discontinued Operations	Mar. 31, 2012 OneBeacon Runoff Segment, Discontinued Operations
Reinsurance [Line Items]
Number of insurance companies sold														2
Loss from sale of discontinued operations				$ 0	$ 0
Net income (loss) from discontinued operations, net of tax	0.5	(9.3)		0.5	(9.3)							0.5	(9.4)
Net favorable and unfavorable loss reserve development	1.4	(7.6)													0.2	(12.4)
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							1,381.0	1,401.9
Reinsurance recoverable on paid losses	15.6		17.9	14.1		15.6	14.1
Reinsurance recoverables, gross of purchase accounting adjustments							1,960.9
Purchase accounting adjustments							$ 146.8