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Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Expense Amortized      
Beginning of period $ 140.7    
End of period (3) $ 140.0 $ 140.7  
OneBeacon | Performance Shares | Long Term Incentive Plan      
Target options outstanding      
Beginning of period 449,435 517,470 493,421
Shares paid or expired (in shares) (167,300) (181,290) (142,138)
New grants 163,150 154,887 165,800
Assumed forfeitures and cancellations (in shares) 7,234 (41,632) 387
End of period (3) 452,519 449,435 517,470
Expense Amortized      
Beginning of period $ 1.4 $ 3.4 $ 4.0
Payments and deferrals (0.6) (1.5) (1.0)
Assumed forfeitures and cancellations 0.0 (0.1) 0.0
Expense recognized 0.8 (0.4) 0.4
End of period (3) $ 1.6 $ 1.4 $ 3.4
Range Of Performance Cycle From 2013 to 2015 [Member] | Long Term Incentive Plan      
Expense Amortized      
Percentage of payout for minimum growth target percentage 24.30%    
Performance Cycle From 2012 to 2014 | Long Term Incentive Plan      
Expense Amortized      
Percentage of payout for minimum growth target percentage   45.70%  
Range of Performance Cycle from 2011 to 2013 [Member] | Long Term Incentive Plan      
Expense Amortized      
Percentage of payout for minimum growth target percentage     37.10%