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Investments - Schedule of Credit Quality of Available-for-sale Debt Securities With Contractual Maturities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 760,408 $ 649,334
Percentage of Total Fair Value 100.00% 100.00%
Comparable Ratings, AAA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 344,801 $ 317,313
Percentage of Total Fair Value 45.30% 48.90%
Comparable Ratings, AA Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 92,411 $ 129,573
Percentage of Total Fair Value 12.20% 20.00%
Comparable Ratings, A Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 216,074 $ 146,749
Percentage of Total Fair Value 28.40% 22.60%
Comparable Ratings, BBB Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 103,427 $ 51,020
Percentage of Total Fair Value 13.60% 7.80%
Comparable Ratings, BB+ and Below Rating [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 117 $ 1,569
Percentage of Total Fair Value 0.00% 0.20%
Comparable Ratings, No Rating Available [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 3,578 $ 3,110
Percentage of Total Fair Value 0.50% 0.50%