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Investments - Schedule of Credit Quality of Investment Securities With Contractual Maturities or The Issuer of Such Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 884,093 $ 820,438
Percentage of Total Fair Value 100.00% 100.00%
AAA [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 419,914 $ 388,672
Percentage of Total Fair Value 47.50% 47.40%
AA [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 102,686 $ 100,791
Percentage of Total Fair Value 11.60% 12.30%
A [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 232,297 $ 214,503
Percentage of Total Fair Value 26.30% 26.10%
BBB [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 124,880 $ 112,613
Percentage of Total Fair Value 14.10% 13.70%
BB and Below [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 0 $ 494
Percentage of Total Fair Value 0.00% 0.10%
Not Rated Available [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 4,316 $ 3,365
Percentage of Total Fair Value 0.50% 0.40%