Discontinued Operations - Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt	$ 383.3	$ 343.1
Estimate of Fair Value Measurement [Member] | SIG Senior Notes | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt	423.6	437.8
Estimate of Fair Value Measurement [Member] | SIG Preference Shares | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt	252.5	260.0
Book Value | SIG Senior Notes | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt	399.8	399.7
Book Value | SIG Preference Shares | Sirius Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt	$ 250.0	$ 250.0