Discontinued Operations (Details 2) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Jun. 30, 2012	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Assets
Investments	$ 2,273.0	$ 2,707.6
Cash	32.0	54.9		63.0		33.6
Reinsurance recoverable on unpaid losses	41.3	2,167.5	2,040.7	1,939.3	1,825.9	1,893.2
Reinsurance recoverable on paid losses	2.0	16.5
Premiums receivable	256.6	230.9
Deferred acquisition costs	132.5	123.5
Net deferred tax asset	129.7	93.6
Other assets	255.7	269.2
Total assets	5,594.5	5,821.6
Liabilities
Loss and loss adjustment expense reserves	911.6	3,358.6	3,175.3	3,159.1	3,131.5	3,295.5
Unearned premiums	610.7	528.0
Ceded reinsurance payable	2.9	23.4
Other liabilities	332.7	397.7
Total liabilities	4,531.0	4,707.7
Remaining purchase accounting fair value adjustments relating to the OneBeacon Acquisition	153.4	163.3
Gross of the purchase accounting adjustments, reinsurance recoverable on unpaid losses	2,366.3	2,110.0
Gross of the purchase accounting adjustments, LAE reserves	36.7
Runoff
Assets
Reinsurance recoverable on unpaid losses	1,956.6
Reinsurance recoverable on paid losses	17.6
Liabilities
Loss and loss adjustment expense reserves	36.7
Other liabilities	140.7
AutoOne business
Liabilities
Net assets held for sale				25.0
Runoff and AutoOne business
Assets
Investments	377.3	111.8
Cash		5.5
Reinsurance recoverable on unpaid losses	1,956.6	0
Reinsurance recoverable on paid losses	17.6	0
Premiums receivable	13.6	8.8
Deferred acquisition costs		2.2
Net deferred tax asset	6.1	1.9
Other assets	17.0	2.4
Total assets	2,388.2	132.6
Liabilities
Loss and loss adjustment expense reserves	2,212.9	64.7
Unearned premiums	0.6	34.1
Ceded reinsurance payable	19.5	0
Other liabilities	155.2	8.8
Total liabilities	2,388.2	107.6
Net assets held for sale		$ 25.0