Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 751.2	$ 677.5
SIG Preference Shares	250.0	250.0
2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	274.7	0
2003 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	0	269.8
Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
SIG Preference Shares	257.5	217.5
Fair value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	282.4	0
Fair value | 2003 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	0	277.4
Fair value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	441.9	418.6
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	0
Carrying Value | 2003 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	0	269.8
Carrying Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 399.4	$ 399.3