Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended											1 Months Ended	12 Months Ended							12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 OneBeacon Runoff	Dec. 31, 2012 OneBeacon	Dec. 31, 2011 OneBeacon	Dec. 31, 2010 OneBeacon	Dec. 31, 2012 Discontinued Operations	Dec. 31, 2011 Discontinued Operations	Dec. 31, 2010 Discontinued Operations	Dec. 31, 2012 Loss attributable to change in discount rate OneBeacon Runoff	Oct. 31, 2011 Esurance	Dec. 31, 2012 Esurance	Dec. 31, 2011 Esurance	Dec. 31, 2010 Esurance	Dec. 31, 2012 Esurance Discontinued Operations	Dec. 31, 2011 Esurance Discontinued Operations	Dec. 31, 2010 Esurance Discontinued Operations	Feb. 22, 2012 AutoOne Entity	Dec. 31, 2012 AutoOne Discontinued Operations	Dec. 31, 2011 AutoOne Discontinued Operations	Dec. 31, 2012 OneBeacon Runoff Contract	Dec. 31, 2012 OneBeacon Runoff GRC Cover	Dec. 31, 2012 OneBeacon Runoff NICO Cover	May 31, 2001 OneBeacon Runoff General Reinsurance Corporation GRC Cover	Dec. 31, 2012 OneBeacon Runoff Discontinued Operations	Dec. 31, 2011 OneBeacon Runoff Discontinued Operations	Dec. 31, 2010 OneBeacon Runoff Discontinued Operations
Reinsurance [Line Items]
Gain (loss) on sale of discontinued operation, net of tax												$ (91.5)				$ (91.0)	$ (19.2)	$ 0		$ 677.5	$ 0	$ 677.5	$ 0	$ 0	$ 677.5	$ 0		$ 0.5	$ (19.2)
Number of insurance companies sold																											2
Net (loss) income from discontinued operations																(115.0)	621.6	(30.1)
Loss and LAE incurred relating prior year losses									41.9	76.7	93.1		7.4	29.8	36.0																			(40.0)	(26.7)	(23.1)
Workers compensation discount amount																																		4.50%	3.50%
Workers compensation discount percent																																		15.2
Net loss from other discontinued operations, net of tax	0.5	(106.8)	0.5	(9.2)	647.6	(30.1)	(2.4)	6.5	(24.0)	(36.7)	(30.1)					(24.0)	(36.7)	(30.1)	(9.0)
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																																	400.0
Loss reserves ceded at acquisition																																	170.0
Reinsurance recoverable on paid and unpaid losses	447.0				2,537.8				447.0	2,537.8			110.7	2,184.1																				1,401.9
Reinsurance recoverable on paid losses	17.9				30.5				17.9	30.5			3.3			15.6	0																	15.6
Reinsurance recoverables																																		1,840.8
Purchase accounting adjustments																																		150.1
Amount of held for sale to be reinsured at closing																																		$ 36.7