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Investments - Schedule of Credit Quality of Investment Securities With Contractual Maturities or The Issuer of Such Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 649,334 $ 589,363
Percentage of Total Fair Value 100.00% 100.00%
Comparable Ratings, AAA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 135,237 $ 131,260
Percentage of Total Fair Value 20.80% 22.30%
Comparable Ratings, AA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 292,496 $ 275,480
Percentage of Total Fair Value 45.10% 46.70%
Comparable Ratings, A Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 134,505 $ 107,418
Percentage of Total Fair Value 20.70% 18.20%
Comparable Ratings, BBB Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 80,566 $ 67,263
Percentage of Total Fair Value 12.40% 11.40%
Comparable Ratings, BB and Below Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 2,919 $ 3,444
Percentage of Total Fair Value 0.40% 0.60%
Comparable Ratings, No Rating Available [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 3,611 $ 4,498
Percentage of Total Fair Value 0.60% 0.80%