Discontinued Operations (Assets and Liabilities Classified as Held for Sale) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets held for sale
Fixed maturity investments, at fair value		$ 2,081.1
Common equity securities, at fair value		0.0
Total investments	$ 3,380.7	2,714.4
Goodwill and intangible assets	62.1	45.2
Liabilities held for sale
Debt	23.8	12.7
Liabilities held for sale		2,569.3
Sirius Group, Tranzact, and Star and Shield | Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale		3,606.4
Assets held for sale
Fixed maturity investments, at fair value		2,175.7
Short-term investments, at amortized cost (which approximates fair value)		112.3
Common equity securities, at fair value		188.7
Other long-term investments		150.5
Total investments		2,627.2
Cash		70.5
Reinsurance recoverable on unpaid losses		179.8
Insurance premiums receivable		229.8
Deferred acquisition costs		96.3
Deferred tax asset		126.7
Ceded unearned insurance and reinsurance premiums		44.2
Accounts receivable on unsettled investment sales		1.4
Goodwill and intangible assets		1.2
Accrued investment income		11.3
Other assets		218.0
Assets held for sale		3,606.4
Liabilities held for sale
Loss and loss adjustment expense reserves		1,370.6
Unearned insurance and reinsurance premiums		576.3
Debt		273.2
Accrued incentive compensation		44.3
Funds held under insurance contracts		153.0
Other liabilities		151.9
Liabilities held for sale		2,569.3
Net assets held for sale		$ 1,037.1
Connecticut | Sirius Group, Tranzact, and Star and Shield | Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	3.3
Other expense	3.7
Assets held for sale
Assets held for sale	$ 3.3