Investments - Summary of Fixed Maturities and Equity Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 320,717	$ 332,843
Less Than 12 Months, Unrealized Losses	6,391	7,069
More Than 12 Months, Fair Value	6,088	5,834
More Than 12 Months, Unrealized Losses	431	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	40,432	37,730
Less Than 12 Months, Unrealized Losses	1,234	1,280
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	33,890	40,739
Less Than 12 Months, Unrealized Losses	594	802
More Than 12 Months, Fair Value	705	710
More Than 12 Months, Unrealized Losses	10	9
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	69,055	80,181
Less Than 12 Months, Unrealized Losses	1,589	2,127
More Than 12 Months, Fair Value	4,839	4,707
More Than 12 Months, Unrealized Losses	338	472
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	173,073	168,772
Less Than 12 Months, Unrealized Losses	2,857	2,728
More Than 12 Months, Fair Value	108	417
More Than 12 Months, Unrealized Losses		3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	4,267	5,421
Less Than 12 Months, Unrealized Losses	117	$ 132
More Than 12 Months, Fair Value	436
More Than 12 Months, Unrealized Losses	$ 83