Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 676.2	$ 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	262.5	257.5
Fair Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	287.8	282.4
Fair Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	454.0	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	$ 399.4	$ 399.4