Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 676.3	$ 751.2
SIG Preference Shares	250.0	250.0
2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	274.7	274.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
SIG Preference Shares	257.5	257.5
Fair Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	273.6	282.4
Fair Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	440.0	441.9
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
SIG Preference Shares	250.0	250.0
Carrying Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	274.7	274.7
Carrying Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 399.5	$ 399.4