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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
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale-securities, Fair Value/Carrying Value $ 118,038 $ 190,723
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 12,021  
One to two years 9,145  
Two to three years 7,345  
Three to five years 4,032  
Available-for-sale-securities, Fair Value/Carrying Value 32,543 66,412
Municipal Fixed-Rate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 891  
One to two years 826  
Two to three years 212  
Three to five years 953  
Available-for-sale-securities, Fair Value/Carrying Value 2,882 11,774
Asset-Backed Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 143  
One to two years 2,367  
Two to three years 2,245  
Three to five years 810  
Five to ten years 158  
More than ten years 803  
Available-for-sale-securities, Fair Value/Carrying Value 6,526 10,206
Mortgage/Agency-Backed Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Three to five years 356  
Five to ten years 1,144  
More than ten years 4,009  
Available-for-sale-securities, Fair Value/Carrying Value 5,509 13,004
U.S. Government Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 3,073  
One to two years 5,960  
Two to three years 3,568  
Three to five years 1,702  
Available-for-sale-securities, Fair Value/Carrying Value 14,303 29,767
Foreign Government Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Two to three years 730  
Available-for-sale-securities, Fair Value/Carrying Value $ 730 $ 3,730