Investments - Summary of Fixed Maturities and Equity Securities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 448,838	$ 174,277
Less Than 12 Months, Unrealized Losses	10,953	1,756
More Than 12 Months, Fair Value	263,311	203,737
More Than 12 Months, Unrealized Losses	13,905	4,956
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	65,701	24,024
Less Than 12 Months, Unrealized Losses	1,017	287
More Than 12 Months, Fair Value	48,064	33,987
More Than 12 Months, Unrealized Losses	2,771	990
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	58,935	10,223
Less Than 12 Months, Unrealized Losses	1,586	120
More Than 12 Months, Fair Value	16,385	14,127
More Than 12 Months, Unrealized Losses	861	295
Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	179,587	35,204
Less Than 12 Months, Unrealized Losses	5,150	253
More Than 12 Months, Fair Value	39,074	31,561
More Than 12 Months, Unrealized Losses	2,473	1,004
Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	144,615	100,534
Less Than 12 Months, Unrealized Losses	3,200	817
More Than 12 Months, Fair Value	159,788	124,062
More Than 12 Months, Unrealized Losses	$ 7,800	2,667
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value		4,292
Less Than 12 Months, Unrealized Losses		$ 279