Discontinued Operations (Net Assets Held for Sale) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Assets held for sale
Fixed maturity investments, at fair value	$ 4,854.4	$ 5,196.2
Cash	411.5	462.4
Reinsurance recoverable on unpaid losses	410.8	429.1	2,465.4	2,507.3
Reinsurance recoverable on paid losses	15.6	17.9
Insurance premiums receivable	717.6	556.3
Deferred acquisition costs	190.4	195.3
Deferred tax asset	539.1	569.6
Other assets	416.5	503.0
Total assets	12,611.5	12,895.4
Liabilities held for sale
Loss and loss adjustment expense reserves	3,100.6	3,168.9	5,510.8	5,702.3
Unearned insurance premiums	1,064.9	924.1
Ceded reinsurance payable	175.7	116.5
Other liabilities	374.4	452.8
Total liabilities	8,329.5	8,637.2
Segment, Discontinued Operations
Assets held for sale
Fixed maturity investments, at fair value	296.9	338.1
Cash	0	0
Reinsurance recoverable on unpaid losses	1,814.1	1,840.8
Reinsurance recoverable on paid losses	14.1	15.6
Insurance premiums receivable	9.9	11.0
Deferred acquisition costs	0	0
Deferred tax asset	4.2	5.1
Other assets	16.0	16.2
Total assets	2,155.2	2,226.8
Liabilities held for sale
Loss and loss adjustment expense reserves	1,980.2	2,052.6
Unearned insurance premiums	0.5	0.5
Ceded reinsurance payable	23.5	21.9
Other liabilities	151.0	151.8
Total liabilities	2,155.2	2,226.8
Net assets held for sale	$ 0	$ 0