Fair Value Measurements - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 620,781	$ 662,276
States and Political Subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	433,734	423,754
Corporate Bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	159,659	170,573
Commercial Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	290	37,683
U.S. Agency-Based Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	12,136	14,471
US Treasury Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	7,029	7,599
US Government-sponsored Enterprises Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	5,819	5,781
Asset-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	2,114	2,415
Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	7,029	7,599
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	7,029	7,599
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	613,752	654,677
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	433,734	423,754
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	159,659	170,573
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	290	37,683
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	12,136	14,471
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	5,819	5,781
Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 2,114	$ 2,415