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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 ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 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 $ 5,319,900,000 $ 4,938,200,000 $ 4,938,200,000  
Less reinsurance recoverable     (560,700,000) $ (354,800,000)
Net loss and LAE at the start of the year 4,759,200,000 4,583,400,000 4,583,400,000  
Net loss and LAE expenses assumed 0 5,700,000.0 (80,100,000)  
Provision for losses and LAE for claims incurred:        
Current year 354,400,000 379,000,000 1,705,400,000  
Prior years (26,200,000) (21,600,000) (129,300,000)  
Total incurred 328,200,000 357,400,000 1,576,100,000  
Losses and LAE payments for claims incurred:        
Current year (14,800,000) (8,500,000) (241,000,000)  
Prior years (383,600,000) (286,100,000) (981,800,000)  
Total paid (398,400,000) (294,600,000) (1,222,800,000)  
Foreign exchange losses/(gains) 41,100,000 (6,400,000) (97,400,000)  
Net losses and LAE reserves at period end 4,730,100,000 4,645,500,000 4,759,200,000  
Plus reinsurance recoverable on unpaid losses at period end 635,800,000 366,000,000 560,700,000  
Provision for losses and LAE at the end of the relevant period $ 5,365,900,000 $ 5,011,500,000 $ 5,319,900,000