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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, 2016
Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 589,363 $ 441,104
Percentage of Total Fair Value 100.00% 100.00%
Comparable Ratings, AAA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 131,260 $ 103,097
Percentage of Total Fair Value 22.30% 23.40%
Comparable Ratings, AA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 275,480 $ 189,600
Percentage of Total Fair Value 46.70% 43.00%
Comparable Ratings, A Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 107,418 $ 83,850
Percentage of Total Fair Value 18.20% 19.00%
Comparable Ratings, BBB Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 67,263 $ 41,408
Percentage of Total Fair Value 11.40% 9.40%
Comparable Ratings, BB and Below Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 3,444 $ 4,261
Percentage of Total Fair Value 0.60% 1.00%
Comparable Ratings, No Rating Available [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 4,498 $ 18,888
Percentage of Total Fair Value 0.80% 4.20%