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Reserves for Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses ("LAE") Reserves (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]        
Provision for losses and LAE at the start of the year $ 5,319.9 $ 4,938.2 $ 4,938.2  
Net loss and LAE at the start of the year 4,759.2 4,583.4 4,583.4  
Net loss and LAE expenses assumed (125.7) 5.7 (80.1)  
Provision for losses and LAE for claims incurred:        
Current year 1,543.3 1,267.0 1,705.4  
Prior years (92.8) (78.2) (129.3)  
Total incurred 1,450.5 1,188.8 1,576.1  
Losses and LAE payments for claims incurred:        
Current year (150.4) (62.1) (241.0)  
Prior years (947.7) (853.1) (981.8)  
Total paid (1,098.1) (915.2) (1,222.8)  
Foreign exchange losses/(gains) 135.2 (35.9) (97.4)  
Net losses and LAE reserves at period end 5,121.1 4,826.8 4,759.2  
Plus reinsurance recoverable on unpaid losses at period end 1,369.5 419.8 560.7 $ 354.8
Provision for losses and LAE at the end of the relevant period $ 6,490.6 $ 5,246.6 $ 5,319.9