Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 748.5	$ 746.6
OneBeacon | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	274.7	274.7
OneBeacon | Book Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	274.7	274.7
OneBeacon | Estimate of Fair Value Measurement [Member] | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	280.4	286.0
Sirius Group | Book Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	399.7	399.7
Sirius Group | Book Value | SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	250.0	250.0
Sirius Group | Estimate of Fair Value Measurement [Member] | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	437.8	437.8
Sirius Group | Estimate of Fair Value Measurement [Member] | SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	261.8	260.0
Tranzact [Member] | Tranzact Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term Line of Credit	70.6	68.7
Tranzact [Member] | Estimate of Fair Value Measurement [Member] | Tranzact Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term Line of Credit	$ 70.2	$ 68.7