Investments - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total investments at fair value	$ 156,278	$ 122,275
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	156,278	122,275
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	4,050	8,863
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	115,811	76,757
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	36,417	36,655
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	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	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	122,681	114,901
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	4,050	1,489
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	82,214	76,757
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	$ 36,417	36,655
Fair Value Measurements on Recurring Basis [Member] | Level 3 [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total investments at fair value		7,374
Fair Value Measurements on Recurring Basis [Member] | Level 3 [Member] | Municipal Bonds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total investments at fair value		$ 7,374