Debt (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended	1 Months Ended					1 Months Ended
	May 31, 2003	Sep. 30, 2011	Apr. 20, 2011 Tender Offer	Apr. 20, 2011 OneBeacon U.S. Holdings, Inc. ("OBH")	Sep. 30, 2012 OneBeacon U.S. Holdings, Inc. ("OBH")	Dec. 31, 2011 OneBeacon U.S. Holdings, Inc. ("OBH")	May 31, 2003 OneBeacon U.S. Holdings, Inc. ("OBH")	Apr. 20, 2011 OneBeacon U.S. Holdings, Inc. ("OBH") Tender Offer
Debt
Senior unsecured notes, at face value					$ 269.9	$ 269.9	$ 700.0
Unamortized original issue discount					(0.1)	(0.2)
Total debt					269.8	269.7
Percentage of par value at which debt was issued	99.70%
Cash received from debt issuance	693.4
Interest rate (as a percent)					5.875%
Debt issue costs							7.3
Underwriting discount							4.5
Effective yield (as a percent)					6.00%
Aggregate principal amount of senior notes purchased and retired			150.0	150.0
Aggregate purchase price of senior notes purchased and retired		161.6						161.6
Loss on retirement of debt								$ (12.0)