Third Party Reinsurance (Written and Earned Premiums) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Written premiums:
Direct	$ 1,390.1	$ 1,218.7	$ 1,358.3
Assumed	1,047.9	1,037.7	1,013.3
Gross written premiums	2,438.0	2,256.4	2,371.6
Ceded	(311.1)	(278.0)	(338.1)
Net written premiums	2,126.9	1,978.4	2,033.5
Earned premiums:
Direct	1,328.2	1,164.4	1,360.4
Assumed	1,041.1	1,032.1	990.4
Gross earned premiums	2,369.3	2,196.5	2,350.8
Ceded	(305.7)	(272.0)	(321.8)
Net earned premiums	2,063.6	1,924.5	2,029.0
Losses and LAE:
Direct	784.4	631.8	736.5
Assumed	553.5	637.0	729.5
Gross losses and LAE	1,337.9	1,268.8	1,466.0
Ceded	(144.0)	(94.5)	(249.4)
Total incurred losses and LAE	1,193.9	1,174.3	1,216.6
OneBeacon
Written premiums:
Direct	1,204.0	1,079.2	1,236.7
Assumed	55.2	49.1	55.8
Gross written premiums	1,259.2	1,128.3	1,292.5
Ceded	(80.0)	(65.6)	(124.8)
Net written premiums	1,179.2	1,062.7	1,167.7
Earned premiums:
Direct	1,158.3	1,035.9	1,242.5
Assumed	52.8	42.3	60.7
Gross earned premiums	1,211.1	1,078.2	1,303.2
Ceded	(79.1)	(66.0)	(122.1)
Net earned premiums	1,132.0	1,012.2	1,181.1
Losses and LAE:
Direct	687.5	551.8	677.1
Assumed	29.6	9.2	45.1
Gross losses and LAE	717.1	561.0	722.2
Ceded	(67.1)	(12.7)	(36.6)
Total incurred losses and LAE	650.0	548.3	685.6
Sirius Group
Written premiums:
Direct	186.1	139.5	121.6
Assumed	992.7	988.6	957.5
Gross written premiums	1,178.8	1,128.1	1,079.1
Ceded	(231.1)	(212.4)	(213.3)
Net written premiums	947.7	915.7	865.8
Earned premiums:
Direct	169.9	128.5	117.9
Assumed	988.3	989.8	929.7
Gross earned premiums	1,158.2	1,118.3	1,047.6
Ceded	(226.6)	(206.0)	(199.7)
Net earned premiums	931.6	912.3	847.9
Losses and LAE:
Direct	96.9	80.0	59.4
Assumed	523.9	627.8	684.4
Gross losses and LAE	620.8	707.8	743.8
Ceded	(76.9)	(81.8)	(212.8)
Total incurred losses and LAE	$ 543.9	$ 626.0	$ 531.0