Fair Value of Financial Instruments (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	$ 669.0	$ 818.8
Preference Shares	250.0	250.0
Sirius Group | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	399.3	399.2
Preference Shares	250.0	250.0
OneBeacon U.S. Holdings, Inc. ("OBH") | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	269.7	419.6
Sirius Group | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	403.7	390.9
Preference Shares	220.0	212.5
OneBeacon U.S. Holdings, Inc. ("OBH") | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	$ 276.6	$ 435.1