Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended									1 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	Jun. 30, 2012	Mar. 31, 2012	Jan. 02, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Oct. 31, 2011 Esurance	Feb. 22, 2012 AutoOne Entity	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 Segment, Discontinued Operations [Member] AutoOne	Sep. 30, 2012 Segment, Discontinued Operations [Member] AutoOne	Sep. 30, 2012 Loss attributable to change in discount rate [Member]	Sep. 30, 2012 OneBeacon Runoff [Member]
Discontinued Operations
Gain (loss) on sale of discontinued operation, net of tax	$ (91.0)	$ (18.2)	$ (91.0)	$ (18.2)								$ 677.5		$ (91.0)	$ (18.2)	$ (91.0)	$ 18.2		$ 0.1	$ 0.5		$ (91.5)
Number of insurance companies sold													2
Assets held for sale
Fixed maturity investments, at fair value	4,912.4		4,912.4					6,221.9						377.3		377.3		111.8
Cash	549.2		549.2					705.4						0		0		5.5
Reinsurance recoverable on unpaid losses	356.8	2,581.2	356.8	2,581.2		2,369.4		2,507.3		2,193.0	2,344.0			1,956.6		1,956.6		0
Reinsurance recoverable on paid losses	14.8		14.8					30.5						17.6		17.6		0
Insurance premiums receivable	678.1		678.1					489.2						13.6		13.6		8.8
Deferred acquisition costs	211.3		211.3				5.6	187.0						0		0		2.2
Deferred Income Tax Assets	555.0		555.0					536.9						6.1		6.1		1.9
Other assets	626.5		626.5					681.9						17.0		17.0		2.4
Total assets held for sale	13,468.8		13,468.8					14,064.0						2,388.2		2,388.2		132.6
Liabilities held for sale
Loss and loss adjustment expense reserves	3,059.8	5,882.0	3,059.8	5,882.0	5,329.8			5,702.3	5,642.4		5,736.8			2,212.9		2,212.9		64.7
Unearned insurance premiums	1,067.1		1,067.1					846.9						0.6		0.6		34.1
Ceded reinsurance payable	138.6		138.6					134.6						19.5		19.5		0
Other liabilities	461.8		461.8					527.8						155.2		155.2		8.8
Total liabilities	9,090.4		9,090.4					9,396.1						2,388.2		2,388.2		107.6
Net assets held for sale														0		0		25.0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent														(106.8)	(30.2)	(115.5)	(26.0)					(100.5)
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)