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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, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 819,861 $ 855,284
Percentage of Total Fair Value 100.00% 100.00%
AAA [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 337,462 $ 372,442
Percentage of Total Fair Value 41.20% 43.60%
AA [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 89,681 $ 99,103
Percentage of Total Fair Value 10.90% 11.60%
A [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 230,290 $ 238,766
Percentage of Total Fair Value 28.10% 27.90%
BBB [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 160,662 $ 143,889
Percentage of Total Fair Value 19.60% 16.80%
BB and Below [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 233 $ 0
Percentage of Total Fair Value 0.00% 0.00%
No Rating Available [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 1,533 $ 1,084
Percentage of Total Fair Value 0.20% 0.10%