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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
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 855,284 $ 820,438
Percentage of Total Fair Value 100.00% 100.00%
AAA [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 372,442 $ 388,672
Percentage of Total Fair Value 43.60% 47.40%
AA [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 99,103 $ 100,791
Percentage of Total Fair Value 11.60% 12.30%
A [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 238,766 $ 214,503
Percentage of Total Fair Value 27.90% 26.10%
BBB [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 143,889 $ 112,613
Percentage of Total Fair Value 16.80% 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%
No Rating Available [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 1,084 $ 3,365
Percentage of Total Fair Value 0.10% 0.40%