Third Party Reinsurance (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Reinsurance [Line Items]
Reinsurance recoverable on paid losses	$ 14.8		$ 30.5
Reinsurance recoverable on unpaid losses	356.8	2,369.4	2,507.3	2,581.2	2,193.0	2,344.0
Purchase accounting adjustments	153
Amount Retained Under Reinsurance Cover	49.3
OneBeacon
Reinsurance [Line Items]
Number of ratings in A.M. Best ratings	15
Reinsurance recoverable on paid losses	2.0
Reinsurance recoverable on unpaid losses	41.3
Reinsurance recoverables	1,449.2
Number of Contracts Seller Caused One Beacon to Purchase Prior to Acquisition	2
OneBeacon | Property catastrophe reinsurance program
Reinsurance [Line Items]
Amount of loss on a single catastrophe retained, first layer	25.0
Amount of loss on a single catastrophe reinsured, second layer	155.0
Property catastrophe reinsurance program second layer first retention percentage	55.00%
Second retention percentage of loss on a single catastrophe reinsured, second layer	15.00%
Property catastrophe reinsurance program second layer third retention percentage	10.00%
Amount of loss on a single catastrophe retained, third layer	180.0
OneBeacon | Property catastrophe reinsurance program | Minimum
Reinsurance [Line Items]
Amount of loss on a single catastrophe reinsured, first retention slab for second layer	25.0
Amount of loss on a single catastrophe reinsured, second retention slab for second layer	40.0
Property catastrophe reinsurance program third retention slab for second layer	80.0
OneBeacon | Property catastrophe reinsurance program | Maximum
Reinsurance [Line Items]
Amount of loss on a single catastrophe reinsured, first retention slab for second layer	40.0
Amount of loss on a single catastrophe reinsured, second retention slab for second layer	80.0
Property catastrophe reinsurance program third retention slab for second layer	180.0
OneBeacon | Hannover Ruversich [Member]
Reinsurance [Line Items]
Reinsurance recoverables	6.4
Percentage of total paid and unpaid reinsurance recoverables	15.00%
OneBeacon | Hartford Steam Boiler [Member]
Reinsurance [Line Items]
Reinsurance recoverables	4.6
Percentage of total paid and unpaid reinsurance recoverables	11.00%
OneBeacon | Munich Reinsurance America
Reinsurance [Line Items]
Reinsurance recoverables	4.5
Percentage of total paid and unpaid reinsurance recoverables	10.00%
OneBeacon | Platinum Underwriters Re [Member]
Reinsurance [Line Items]
Reinsurance recoverables	4.1
Percentage of total paid and unpaid reinsurance recoverables	9.00%
OneBeacon | Transatlantic Reinsurance [Member]
Reinsurance [Line Items]
Reinsurance recoverables	$ 2.0
Percentage of total paid and unpaid reinsurance recoverables	5.00%