Third-Party Reinsurance (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Written premiums:
Direct	$ 1,217.1	$ 1,623.1	$ 2,079.7
Assumed	1,036.8	1,015.2	947.8
Gross written premiums	2,253.9	2,638.3	3,027.5
Ceded	(275.3)	(613.5)	(402.9)
Net written premiums	1,978.6	2,024.8	2,624.6
Earned premiums:
Direct	1,267.0	1,766.6	2,128.9
Assumed	1,032.9	993.9	994.3
Gross earned premiums	2,299.9	2,760.5	3,123.2
Ceded	(372.1)	(508.7)	(405.6)
Net earned premiums	1,927.8	2,251.8	2,717.6
Losses and LAE:
Direct	1,231.1	936.5	1,089.5
Assumed	663.9	822.8	533.0
Gross losses and LAE	1,895.0	1,759.3	1,622.5
Ceded	(688.1)	(370.1)	(198.4)
Total incurred losses and LAE	1,206.9	1,389.2	1,424.1
OneBeacon
Written premiums:
Direct	1,077.6	1,501.5	1,972.4
Assumed	48.2	57.7	58.6
Gross written premiums	1,125.8	1,559.2	2,031.0
Ceded	(62.9)	(400.2)	(213.2)
Net written premiums	1,062.9	1,159.0	1,817.8
Earned premiums:
Direct	1,138.5	1,648.7	2,021.7
Assumed	43.1	64.2	66.8
Gross earned premiums	1,181.6	1,712.9	2,088.5
Ceded	(166.1)	(309.0)	(229.7)
Net earned premiums	1,015.5	1,403.9	1,858.8
Losses and LAE:
Direct	1,151.1	877.1	1,020.7
Assumed	36.1	138.4	40.8
Gross losses and LAE	1,187.2	1,015.5	1,061.5
Ceded	(606.3)	(157.3)	(56.2)
Total incurred losses and LAE	580.9	858.2	1,005.3
OneBeacon | OneBeacon Commercial Lines
Written premiums:
Ceded		(262.2)
Earned premiums:
Net earned premiums		165.0
Losses and LAE:
Total incurred losses and LAE		86.5
Sirius
Written premiums:
Direct	139.5	121.6	107.3
Assumed	988.6	957.5	889.2
Gross written premiums	1,128.1	1,079.1	996.5
Ceded	(212.4)	(213.3)	(189.7)
Net written premiums	915.7	865.8	806.8
Earned premiums:
Direct	128.5	117.9	107.2
Assumed	989.8	929.7	927.5
Gross earned premiums	1,118.3	1,047.6	1,034.7
Ceded	(206.0)	(199.7)	(175.9)
Net earned premiums	912.3	847.9	858.8
Losses and LAE:
Direct	80.0	59.4	68.8
Assumed	627.8	684.4	492.2
Gross losses and LAE	707.8	743.8	561.0
Ceded	(81.8)	(212.8)	(142.2)
Total incurred losses and LAE	$ 626.0	$ 531.0	$ 418.8