Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 677.5	$ 676.4
Fair Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	283.9	269.8
Fair Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	438.2	438.1
Fair Value | SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	262.5	260.0
Book Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	274.7	274.7
Book Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	399.6	399.6
Book Value | SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 250.0	$ 250.0