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Retroactive Insurance (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Liabilities for Guarantees on Long-Duration Contracts [Line Items]            
Net premiums earned $ 223,591 $ 180,353 $ 410,750 $ 363,256    
Net losses and loss adjustment expenses 161,728 115,550 291,515 234,701    
Reserve for losses and loss adjustment expenses 2,078,817 $ 1,991,865 2,078,817 $ 1,991,865 $ 2,048,381 $ 1,993,428
Retroactive Insurance Contract            
Liabilities for Guarantees on Long-Duration Contracts [Line Items]            
Net premiums earned 26,600   26,600      
Net losses and loss adjustment expenses 25,400   25,400      
Reserve for losses and loss adjustment expenses 600   600      
Retroactive Insurance Contract, Prospective Coverage            
Liabilities for Guarantees on Long-Duration Contracts [Line Items]            
Net premiums earned 7,900   7,900      
Retroactive Insurance Contract, Retroactive Coverage            
Liabilities for Guarantees on Long-Duration Contracts [Line Items]            
Net premiums earned $ 18,700   $ 18,700