Fair Value Measurements - Summary of Carrying Values and Corresponding Fair Values for Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities-held-to-maturity	$ 603,295	$ 568,931
Securities held to maturity, carrying value	594,776	562,434
Fixed maturity securities-available-for-sale	468,690	479,097
Equity securities	2,044	33
Cash and cash equivalents	39,353	58,936
Short-term investments	32,804	29,580
Other investments	8,956	13,330
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities held to maturity, carrying value	594,776	562,434
Fixed maturity securities-available-for-sale	468,690	479,097
Equity securities	2,044	33
Cash and cash equivalents	39,353	58,936
Short-term investments	32,804	29,580
Other investments	$ 8,956	$ 13,330