Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended						3 Months Ended		9 Months Ended			9 Months Ended						3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2013 Segment, Discontinued Operations	Sep. 30, 2012 Segment, Discontinued Operations	Sep. 30, 2013 Segment, Discontinued Operations	Sep. 30, 2012 Segment, Discontinued Operations	Dec. 31, 2012 Segment, Discontinued Operations	Sep. 30, 2013 Segment, Discontinued Operations OneBeacon Runoff Contract	Dec. 31, 2012 Segment, Discontinued Operations OneBeacon Runoff	May 31, 2001 Segment, Discontinued Operations GRC Cover OneBeacon Runoff	May 31, 2001 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	Sep. 30, 2012 OneBeacon Runoff Segment, Discontinued Operations	Sep. 30, 2012 OneBeacon Runoff Segment, Discontinued Operations	Sep. 30, 2013 OneBeacon Runoff Segment, Discontinued Operations	Mar. 31, 2013 OneBeacon Runoff Segment, Discontinued Operations	Mar. 31, 2013 Minimum OneBeacon Runoff Segment, Discontinued Operations	Mar. 31, 2013 Maximum OneBeacon Runoff Segment, Discontinued Operations	Mar. 31, 2013 Item one [Member] OneBeacon Runoff Segment, Discontinued Operations	Mar. 31, 2013 Item two [Member] OneBeacon Runoff Segment, Discontinued Operations
Reinsurance [Line Items]
Other revenue	$ 18.2	$ 50.3	$ 46.9	$ 81.0					$ 0.1	$ 0	$ 12.3	$ 0
Number of insurance companies sold																			2
Loss and loss adjustment expense reserves	3,108.1	3,059.8	3,108.1	3,059.8	3,057.9	3,168.9	5,329.8	5,702.3	1,797.1		1,797.1		2,052.6									131.0	166.0	215.0	668.0
Net income (loss) from discontinued operations									0.4	(106.8)	4.8	(115.5)								100.5	100.5
Difference between net loss and LAE and actuarial estimates																										46	17
Loss from sale of discontinued operations	0	(91.0)	0	(91.0)					0	(91.0)	0	(91.0)								91.5	91.5
Workers Compensation Loss and Loss Adjustment Reserves, Discount Amount																				9.0	9.0
Net income (loss) from discontinued operations, net of tax	0.4	(15.8)	4.8	(24.5)					0.4	(15.8)	4.8	(24.5)
Net favorable and unfavorable loss reserve development	(11.6)	(10.1)	(25.6)	(21.2)																	6.5
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														1,292.2	1,401.9
A M Best Rating System, Number of Debt Ratings														16
Reinsurance recoverable on paid losses														21.8
Reinsurance recoverables, gross of purchase accounting adjustments														1,806.7
Purchase accounting adjustments														$ 140.2