Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 676.6	$ 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	257.5	217.5
Fair Value | OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	276.3	277.4
Fair Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	442.9	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