Fair Value Measurements - Summary of Carrying Values and Corresponding Fair Values for Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities-held-to-maturity	$ 610,151	$ 568,931
Securities held to maturity, carrying value	598,658	562,434
Fixed maturity securities-available-for-sale	467,256	479,097
Equity securities	7,801	33
Short-term investments	69,526	29,580
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities-held-to-maturity	610,151	568,931
Fixed maturity securities-available-for-sale	467,256	479,097
Equity securities	7,801	33
Short-term investments	69,526	29,580
Other investments	1,280	13,330
Cash and cash equivalents	80,378	58,936
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities held to maturity, carrying value	598,658	562,434
Fixed maturity securities-available-for-sale	467,256	479,097
Equity securities	7,801	33
Short-term investments	69,526	29,580
Other investments	1,280	13,330
Cash and cash equivalents	$ 80,378	$ 58,936