Financial Instruments - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total investments at fair value	$ 228,795	$ 196,428
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	228,795	196,428
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	629
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	212,567	179,091
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	15,599	17,337
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	20,000
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	20,000
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	208,795	176,428
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	629
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	192,567	159,091
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	$ 15,599	$ 17,337