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Significant Transactions (OneBeacon Personal Lines and Commercial Lines Sales) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 1 Months Ended 3 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Jul. 31, 2010
One Beacon Personal Lines
company
Jun. 30, 2011
One Beacon Personal Lines
Dec. 31, 2010
One Beacon Personal Lines
Dec. 31, 2009
One Beacon Personal Lines
Dec. 31, 2009
One Beacon Commercial Lines
Dec. 31, 2012
One Beacon Commercial Lines
Dec. 31, 2011
One Beacon Commercial Lines
Dec. 31, 2009
One Beacon Commercial Lines
Significant Transactions                      
Number of insurance companies sold       2              
Written percentage       100.00%              
Assumed risk percentage       100.00%              
Number of Attorneys in Fact       2              
Period to provide certain services       3 years              
Sales of unconsolidated affiliates, net of cash sold and held in escrow $ 24.8 $ 1,010.6 $ 188.3 $ 166.6       $ 23.2 $ 0.8 $ 10.2  
Gain including tax benefit           24.6          
Gain on disposal           8.5          
Tax effect of income (loss) on disposal         5.6 16.1          
Premiums written, net 2,126.9 1,978.4 2,033.5       420.0        
Value of renewal rights in premiums                 $ 200.0   $ 490.0