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Financial Instruments - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value $ 161,248 $ 156,278
Fair Value Measurements on Recurring Basis [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 161,248 156,278
Fair Value Measurements on Recurring Basis [Member] | Municipal Bonds [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 830 4,050
Fair Value Measurements on Recurring Basis [Member] | Non-governmental Debt Securities [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 130,340 115,811
Fair Value Measurements on Recurring Basis [Member] | Treasury and Federal Agencies [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 30,078 36,417
Fair Value Measurements on Recurring Basis [Member] | Level 1 [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 31,500 33,597
Fair Value Measurements on Recurring Basis [Member] | Level 1 [Member] | Non-governmental Debt Securities [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 31,500 33,597
Fair Value Measurements on Recurring Basis [Member] | Level 2 [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 129,748 122,681
Fair Value Measurements on Recurring Basis [Member] | Level 2 [Member] | Municipal Bonds [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 830 4,050
Fair Value Measurements on Recurring Basis [Member] | Level 2 [Member] | Non-governmental Debt Securities [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value 98,840 82,214
Fair Value Measurements on Recurring Basis [Member] | Level 2 [Member] | Treasury and Federal Agencies [Member]    
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]    
Total investments at fair value $ 30,078 $ 36,417