Fair Values of Financial Instruments - Summary of Carrying or Reported Values and Corresponding Fair Values for Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held to maturity	$ 639,309	$ 568,931
Securities held to maturity, carrying value	629,668	562,434
Fixed maturity securities—available for sale	465,594	479,097
Equity securities	9,282	33
Short-term investments	25,770	29,580
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held to maturity	639,309	568,931
Fixed maturity securities—available for sale	465,594	479,097
Equity securities	9,282	33
Short-term investments	25,770	29,580
Other investments		13,330
Cash and cash equivalents	55,559	58,936
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held to maturity	629,668	562,434
Fixed maturity securities—available for sale	465,594	479,097
Equity securities	9,282	33
Short-term investments	25,770	29,580
Other investments		13,330
Cash and cash equivalents	$ 55,559	$ 58,936