Investments - Summary of Fixed Maturities and Equity Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 291,101	$ 332,843
Less Than 12 Months, Unrealized Losses	4,118	7,069
More Than 12 Months, Fair Value	40,365	5,834
More Than 12 Months, Unrealized Losses	1,515	484
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	50,877	37,730
Less Than 12 Months, Unrealized Losses	865	1,280
More Than 12 Months, Fair Value	1,919
More Than 12 Months, Unrealized Losses	78
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	19,651	40,739
Less Than 12 Months, Unrealized Losses	366	802
More Than 12 Months, Fair Value	4,601	710
More Than 12 Months, Unrealized Losses	56	9
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	39,754	80,181
Less Than 12 Months, Unrealized Losses	735	2,127
More Than 12 Months, Fair Value	16,426	4,707
More Than 12 Months, Unrealized Losses	689	472
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	176,141	168,772
Less Than 12 Months, Unrealized Losses	1,677	2,728
More Than 12 Months, Fair Value	16,653	417
More Than 12 Months, Unrealized Losses	521	3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	4,678	5,421
Less Than 12 Months, Unrealized Losses	475	$ 132
More Than 12 Months, Fair Value	766
More Than 12 Months, Unrealized Losses	$ 171