Fair Value Measurements - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 603,295	$ 568,931
States and Political Subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	435,792	399,483
Corporate Bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	140,737	144,016
U.S. Agency-Based Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	10,024	10,915
U.S. Treasury Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,780	6,779
US Government-sponsored Enterprises Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	8,016	5,637
Asset-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	1,946	2,031
Commercial Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value		70
Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,780	6,779
Fair Value, Inputs, Level 1 [Member] | U.S. Treasury Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,780	6,779
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	596,515	562,152
Fair Value, Inputs, Level 2 [Member] | States and Political Subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	435,792	399,483
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	140,737	144,016
Fair Value, Inputs, Level 2 [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	10,024	10,915
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	8,016	5,637
Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 1,946	2,031
Fair Value, Inputs, Level 2 [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value		$ 70