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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, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]        
Provision for losses and LAE at the start of the year $ 6,749.5 $ 5,319.9 $ 5,319.9  
Net loss and LAE at the start of the year 5,234.3 4,759.2 4,759.2  
Net loss and LAE expenses (disposed) 0.0 (125.7) (125.5)  
Provision for losses and LAE for claims incurred:        
Current year 1,151.4 1,543.3 2,100.1  
Prior years (99.7) (92.8) (105.4)  
Total incurred 1,051.7 1,450.5 1,994.7  
Losses and LAE payments for claims incurred:        
Current year (93.5) (150.4) (397.5)  
Prior years (1,170.1) (947.7) (1,157.6)  
Total paid (1,263.6) (1,098.1) (1,555.1)  
Foreign exchange losses/(gains) (63.3) 135.2 161.0  
Net losses and LAE reserves at period end 4,959.1 5,121.1 5,234.3  
Plus reinsurance recoverable on unpaid losses at period end 1,767.1 1,369.5 1,515.2 $ 560.7
Provision for losses and LAE at the end of the relevant period $ 6,726.2 $ 6,490.6 $ 6,749.5