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Investments - Contractual Maturities of Corporate Bonds, Municipal Fixed-Rate Bonds, Asset-Backed Bonds, Mortgage/Agency-Backed Bonds, U.S. Government Bonds and Foreign Government Bonds (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale-securities, Fair Value/Carrying Value $ 178,492 $ 190,723
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 33,486  
One to two years 16,124  
Two to three years 21,421  
Three to five years 1,948  
Available-for-sale-securities, Fair Value/Carrying Value 72,979 66,412
Municipal Fixed-Rate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 4,101  
One to two years 1,536  
Two to three years 775  
Three to five years 1,171  
Available-for-sale-securities, Fair Value/Carrying Value 7,583 11,774
Asset-Backed Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
One to two years 850  
Two to three years 5,185  
Three to five years 5,836  
Five to ten years 508  
More than ten years 929  
Available-for-sale-securities, Fair Value/Carrying Value 13,308 10,206
Mortgage/Agency-Backed Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
One to two years 493  
Three to five years 415  
Five to ten years 1,340  
More than ten years 7,279  
Available-for-sale-securities, Fair Value/Carrying Value 9,527 13,004
U.S. Government Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 8,919  
Two to three years 8,367  
Three to five years 4,114  
Available-for-sale-securities, Fair Value/Carrying Value 21,400 29,767
Foreign Government Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 825  
One to two years 600  
Two to three years 1,277  
Available-for-sale-securities, Fair Value/Carrying Value $ 2,702 $ 3,730