v3.8.0.1
Reserve for Loss and Loss Adjustment Expenses (Details) - USD ($)
$ in Thousands
 3 Months Ended 8 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Aug. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Dec. 31, 2016
Insurance [Abstract]              
Reserve for reported loss and LAE           $ 1,898,474 $ 1,617,956
Reserve for losses incurred but not reported (IBNR)           1,466,537 1,278,540
Reserve for loss and LAE $ 3,365,011 $ 2,759,518 $ 2,896,496 $ 2,896,496 $ 2,510,101 3,365,011 $ 2,896,496
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]              
Gross loss and LAE reserves, January 1     2,896,496 2,896,496 2,510,101    
Less: reinsurance recoverable on unpaid losses, January 1     99,936 99,936 71,248    
Net loss and LAE reserves, January 1     2,796,560 2,796,560 2,438,853    
Net incurred losses related to Current year       1,394,623 1,255,493    
Net incurred losses related to Prior years       150,534 41,868    
Net loss and loss adjustment expenses 535,968 466,751   1,545,157 1,297,361    
Net paid losses related to Current year       (400,908) (356,135)    
Net paid losses related to Prior years       (765,049) (722,395)    
Net paid losses       (1,165,957) (1,078,530)    
Effect of foreign exchange movements       48,622 4,764    
Net loss and LAE reserves, September 30 3,224,382 2,662,448   3,224,382 2,662,448    
Reinsurance recoverable on unpaid losses, September 30 140,629 97,070   140,629 97,070    
Gross loss and LAE reserves, September 30 3,365,011 2,759,518   3,365,011 2,759,518    
Loss from catastrophes [Member]              
Segment Reporting Information [Line Items]              
Estimated loss due to Hurricanes Harvey and Irma           $ 20,000  
Operating segments [Member]              
Segment Reporting Information [Line Items]              
Adverse development, net 77,670 12,446   150,534 41,868    
Operating segments [Member] | Diversified Reinsurance segment [Member]              
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]              
Net loss and loss adjustment expenses 172,273 132,396   487,759 395,718    
Segment Reporting Information [Line Items]              
Adverse development, net 7,878 10,408   39,486 37,404    
Operating segments [Member] | AmTrust Reinsurance segment [Member]              
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]              
Net loss and loss adjustment expenses 355,030 334,310   1,047,222 898,703    
Segment Reporting Information [Line Items]              
Adverse development, net 61,127 1,993   100,872 1,524    
Adverse development, net, from one terminated program     $ 16,237        
Other [Member]              
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]              
Net loss and loss adjustment expenses 8,665 45   10,176 2,940    
Segment Reporting Information [Line Items]              
Adverse development, net $ 8,665 $ 45   $ 10,176 $ 2,940