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Investments - Contractual Maturities of Debt Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale debt securities, Fair Value/Carrying Value $ 89,068 $ 62,493
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 11,391  
One to two years 18,406  
Two to three years 10,639  
Three to five years 8,383  
Available-for-sale debt securities, Fair Value/Carrying Value 48,819 32,543
Municipal Fixed-Rate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 490  
One to two years 737  
Two to three years 210  
Three to five years 946  
Available-for-sale debt securities, Fair Value/Carrying Value 2,383 2,882
Asset-Backed Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than one year 4,520  
One to two years 2,459  
Two to three years 2,669  
Three to five years 1,631  
More than ten years 2,011  
Available-for-sale debt securities, Fair Value/Carrying Value 13,290 6,526
Mortgage/Agency-Backed Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Three to five years 2,046  
Five to ten years 974  
More than ten years 5,816  
Available-for-sale debt securities, Fair Value/Carrying Value 8,836 5,509
U.S. Government Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
One to two years 6,149  
Two to three years 2,491  
Three to five years 5,869  
Available-for-sale debt securities, Fair Value/Carrying Value 14,509 14,303
Foreign Government Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Two to three years 1,231  
Available-for-sale debt securities, Fair Value/Carrying Value $ 1,231 $ 730