Investments - Summary of Fixed Maturities and Equity Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 280,993	$ 332,843
Less Than 12 Months, Unrealized Losses	5,312	7,069
More Than 12 Months, Fair Value	8,313	5,834
More Than 12 Months, Unrealized Losses	827	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	41,087	37,730
Less Than 12 Months, Unrealized Losses	934	1,280
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	20,102	40,739
Less Than 12 Months, Unrealized Losses	385	802
More Than 12 Months, Fair Value	696	710
More Than 12 Months, Unrealized Losses	14	9
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	48,822	80,181
Less Than 12 Months, Unrealized Losses	1,195	2,127
More Than 12 Months, Fair Value	6,051	4,707
More Than 12 Months, Unrealized Losses	534	472
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	164,760	168,772
Less Than 12 Months, Unrealized Losses	2,325	2,728
More Than 12 Months, Fair Value	1,196	417
More Than 12 Months, Unrealized Losses	30	3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	6,222	5,421
Less Than 12 Months, Unrealized Losses	473	$ 132
More Than 12 Months, Fair Value	370
More Than 12 Months, Unrealized Losses	$ 249