Financial Instruments - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total investments at fair value	$ 160,656	$ 161,248
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	160,656	161,248
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
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	143,796	130,340
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	16,860	30,078
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	15,000	31,500
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	15,000	31,500
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	145,656	129,748
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
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	128,796	98,840
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	$ 16,860	$ 30,078