Investments - Summary of Fixed Maturities and Equity Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 355,691	$ 174,277
Less Than 12 Months, Unrealized Losses	6,730	1,756
More Than 12 Months, Fair Value	205,799	203,737
More Than 12 Months, Unrealized Losses	8,337	4,956
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	45,031	24,024
Less Than 12 Months, Unrealized Losses	829	287
More Than 12 Months, Fair Value	35,385	33,987
More Than 12 Months, Unrealized Losses	1,613	990
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	32,405	10,223
Less Than 12 Months, Unrealized Losses	536	120
More Than 12 Months, Fair Value	13,908	14,127
More Than 12 Months, Unrealized Losses	507	295
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	123,863	35,204
Less Than 12 Months, Unrealized Losses	2,427	253
More Than 12 Months, Fair Value	29,595	31,561
More Than 12 Months, Unrealized Losses	1,540	1,004
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	154,392	100,534
Less Than 12 Months, Unrealized Losses	2,938	817
More Than 12 Months, Fair Value	126,911	124,062
More Than 12 Months, Unrealized Losses	$ 4,677	2,667
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value		4,292
Less Than 12 Months, Unrealized Losses		$ 279