Fair Values of Financial Instruments - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 568,931	$ 662,276
Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,779	7,599
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	562,152	654,677
States and Political Subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	399,483	423,754
States and Political Subdivisions [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	399,483	423,754
Corporate Bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	144,016	170,573
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	144,016	170,573
Commercial Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	70	37,683
Commercial Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	70	37,683
U.S. Agency-Based Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	10,915	14,471
U.S. Agency-Based Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	10,915	14,471
U.S. Treasury Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,779	7,599
U.S. Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,779	7,599
US Government-sponsored Enterprises Debt Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	5,637	5,781
US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	5,637	5,781
Asset-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	2,031	2,415
Asset-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 2,031	$ 2,415