Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 676.3	$ 751.2
Total non-controlling interests	497.2	526.4
2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	274.7	274.7
Fair Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	272.2	282.4
Fair Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	437.0	441.9
Fair Value | SIG Preference Shares [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Total non-controlling interests	255.0	257.5
Book Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	274.7	274.7
Book Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	399.5	399.4
Book Value | SIG Preference Shares [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Total non-controlling interests	$ 250.0	$ 250.0