Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			6 Months Ended		1 Months Ended	3 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Segment, Discontinued Operations	Jun. 30, 2012 Segment, Discontinued Operations	Jun. 30, 2013 Segment, Discontinued Operations	Jun. 30, 2012 Segment, Discontinued Operations	Dec. 31, 2012 Segment, Discontinued Operations	Jun. 30, 2013 Segment, Discontinued Operations OneBeacon Runoff Contract	Dec. 31, 2012 Segment, Discontinued Operations OneBeacon Runoff	May 30, 2001 Segment, Discontinued Operations GRC Cover OneBeacon Runoff	Jun. 30, 2013 Segment, Discontinued Operations NICO Cover OneBeacon Runoff	May 31, 2001 Segment, Discontinued Operations General Reinsurance Corporation GRC Cover OneBeacon Runoff	Feb. 22, 2012 AutoOne Entity	Jun. 30, 2012 OneBeacon Runoff Segment, Discontinued Operations	Jun. 30, 2012 OneBeacon Runoff Segment, Discontinued Operations
Reinsurance [Line Items]
Other revenue	$ 0.4	$ (0.5)	$ 28.7	$ 30.7		$ 12.2	$ 0	$ 12.2	$ 0		$ 7.8
Number of insurance companies sold																2
Loss from sale of discontinued operations						0	0	0	0
Net gain (loss) income from discontinued operations, net of tax	3.9	0.5	4.4	(8.8)		3.9	0.5	4.4	(8.8)
Net favorable and unfavorable loss reserve development	(15.4)	(3.5)	(14.0)	(11.1)													0.1	12.5
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											1,315.4	1,401.9
A M Best Rating System, Number of Debt Ratings											16
Reinsurance recoverable on paid losses	16.1		16.1		17.9	21.4		21.4		15.6	21.4
Reinsurance recoverables, gross of purchase accounting adjustments											1,879.8
Purchase accounting adjustments											$ 143.5