Fair Value Measurements - Summary of Carrying Values and Corresponding Fair Values for Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities-held-to-maturity	$ 610,551	$ 568,931
Securities held to maturity, carrying value	599,096	562,434
Fixed maturity securities-available-for-sale	462,793	479,097
Equity securities	5,068	33
Short-term investments	46,635	29,580
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities-held-to-maturity	610,551	568,931
Fixed maturity securities-available-for-sale	462,793	479,097
Equity securities	5,068	33
Short-term investments	46,635	29,580
Other investments	7,038	13,330
Cash and cash equivalents	53,967	58,936
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities held to maturity, carrying value	599,096	562,434
Fixed maturity securities-available-for-sale	462,793	479,097
Equity securities	5,068	33
Short-term investments	46,635	29,580
Other investments	7,038	13,330
Cash and cash equivalents	$ 53,967	$ 58,936