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Reinsurance (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Ceded Credit Risk [Line Items]    
Premiums receivable $ 3,928,122 $ 2,894,631
Premium receivable, allowance for credit loss 3,782 5,961
Reinsurance recoverable 3,200,000 2,900,000
Reinsurance recoverable, allowance for credit loss $ 6,605 $ 6,334
Customer Concentration Risk, Customer One | Reinsurance recoverable    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 15.00% 15.30%
Customer Concentration Risk, Customer One | Allowance for reinsurance recoverable    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 12.70% 13.20%
Customer Concentration Risk, Customer Two | Reinsurance recoverable    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 10.20% 10.80%
Customer Concentration Risk, Customer Two | Allowance for reinsurance recoverable    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 7.60% 13.00%
Customer Concentration Risk, Customer Three | Reinsurance recoverable    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 6.10% 6.70%
Customer Concentration Risk, Customer Three | Allowance for reinsurance recoverable    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 5.10% 6.70%
Ceded Credit Risk, Secured | Reinsurer Concentration Risk | Reinsurance recoverable    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 45.20% 45.20%
Ceded Credit Risk, Unsecured | Reinsurer Concentration Risk | Reinsurance recoverable | Standard & Poor's, A- Rating    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 52.90% 53.40%
Ceded Credit Risk, Unsecured | Reinsurer Concentration Risk | Reinsurance recoverable | Standard & Poor's, BBB Rating    
Ceded Credit Risk [Line Items]    
Concentration risk, percentage 1.90% 1.40%