Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 678.1	$ 676.4
Fair Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	281.4	269.8
Fair Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	441.9	438.1
Fair Value | SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	264.4	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