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Significant Accounting Policies (Details) - USD ($)
 6 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Jan. 01, 2020
Schedule Of Significant Accounting Policies [Line Items]      
Decrease to retained earnings for the cumulative after-tax effect of adopting ASC 326 $ (602,583,000) $ (573,619,000)  
Accounting Standards Update [Extensible List] us-gaap:AccountingStandardsUpdate201613Member us-gaap:AccountingStandardsUpdate201613Member  
Accrued interest receivable on available-for-sale securities $ 5,000,000.0    
Premium receivable, allowance for credit loss 500,000 $ 700,000  
Reinsurance, allowance for credit loss $ 0 $ 0  
Cumulative Effect, Period Of Adoption, Adjustment [Member]      
Schedule Of Significant Accounting Policies [Line Items]      
Decrease to retained earnings for the cumulative after-tax effect of adopting ASC 326     $ 600,000
Minimum [Member]      
Schedule Of Significant Accounting Policies [Line Items]      
Short-term investment maturity period 3 months