SCHEDULE IV REINSURANCE (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
OneBeacon
Premiums earned
Gross amount	$ 1,158.3	$ 1,035.9	$ 1,242.5
Ceded to other companies	(79.1)	(66.0)	(122.1)
Assumed from other companies	52.8	42.3	60.7
Net amount	1,132.0	1,012.2	1,181.1
Percent of amount assumed to net	4.70%	4.20%	5.10%
Sirius Group
Premiums earned
Gross amount	169.9	128.5	117.9
Ceded to other companies	(226.6)	(206.0)	(199.7)
Assumed from other companies	988.3	989.8	929.7
Net amount	$ 931.6	$ 912.3	$ 847.9
Percent of amount assumed to net	106.10%	108.50%	109.60%