Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 677.6	$ 677.5
SIG Preference Shares	250.0	250.0
OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	269.8	269.8
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
SIG Preference Shares	240.0	217.5
Fair Value | OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	277.6	277.4
Fair Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	420.0	418.6
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
SIG Preference Shares	250.0	250.0
Carrying Value | OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	269.8	269.8
Carrying Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 399.4	$ 399.3