Financial Instruments - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total investments at fair value		$ 161,248
Total investments at fair value	$ 196,428	156,178
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	196,428	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	179,091	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	17,337	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	20,000	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	20,000	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	176,428	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	159,091	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	$ 17,337	$ 30,078