Discontinued Operations (Assets and Liabilities Classified as Held for Sale) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred Income Tax Assets, Net	$ 112.8	$ 114.6
Assets held for sale
Fixed maturity investments, at fair value	2,639.7	2,422.0
Short-term investments, at amortized cost (which approximates fair value)	211.3	376.8
Common equity securities, at fair value	1,113.9	611.7
Other long-term investments	315.8	331.9
Total investments	4,280.7	3,742.4
Insurance premiums receivable	223.3	241.1
Ceded unearned insurance premiums	29.5	17.8
Accounts receivable on unsettled investment sales	41.9	37.8
Goodwill and intangible assets	375.7	351.2
Other assets	318.7	348.7
Assets held for sale	4,407.0	4,630.6
Deferred acquisition costs	107.6	107.2
Cash Including Restricted Cash	179.3	261.7
Liabilities held for sale
Loss and loss adjustment expense reserves	1,395.8	1,350.0	$ 1,054.3	$ 1,000.0
Unearned insurance premiums	612.6	616.7
Debt	442.4	343.1
Deferred tax liability	7.0	0.0
Accrued incentive compensation	141.7	108.1
Ceded reinsurance payable	30.5	34.2
Funds held under insurance contracts	137.8	81.0
Accounts payable on unsettled investment purchases	0.0	0.5
Other liabilities	264.7	278.4
Liabilities held for sale	2,884.0	3,105.3
Reinsurance recoverable on unpaid losses	186.5	161.7
Reinsurance recoverable on paid losses	7.5	12.2
Sirius Group
Assets held for sale
Total investments	686.2
Cash Including Restricted Cash	143.9	111.5	$ 93.2	$ 93.7
Sirius Group | Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred Income Tax Assets, Net	303.1	341.5
Assets held for sale
Fixed maturity investments, at fair value	2,374.0	2,362.3
Short-term investments, at amortized cost (which approximates fair value)	352.0	494.9
Common equity securities, at fair value	174.4	189.9
Other long-term investments	72.2	95.6
Total investments	2,972.6	3,142.7
Insurance premiums receivable	323.6	306.6
Ceded unearned insurance premiums	87.7	76.2
Accounts receivable on unsettled investment sales	29.0	18.7
Goodwill and intangible assets	10.2	15.2
Other assets	169.0	156.6
Assets held for sale	4,407.0	4,630.6
Deferred acquisition costs	74.6	69.9
Cash Including Restricted Cash	143.9	111.5
Liabilities held for sale
Loss and loss adjustment expense reserves	1,644.4	1,809.8
Unearned insurance premiums	342.2	338.6
Debt	403.5	403.5
Deferred tax liability	263.6	282.8
Accrued incentive compensation	63.2	76.5
Ceded reinsurance payable	67.1	71.5
Funds held under insurance contracts	52.9	57.9
Accounts payable on unsettled investment purchases	0.0	2.1
Other liabilities	47.1	62.6
Liabilities held for sale	2,884.0	3,105.3
Net assets held for sale	1,523.0	1,525.3
Reinsurance recoverable on unpaid losses	283.1	322.2
Reinsurance recoverable on paid losses	10.2	11.4
OneBeacon Runoff | Discontinued Operations
Assets held for sale
Other assets	$ 0.0	$ 58.1