Investments - Summary of Fixed Maturities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 45,794	$ 286,528
Less Than 12 Months, Unrealized Losses	316	6,039
More Than 12 Months, Fair Value	378,772	317,236
More Than 12 Months, Unrealized Losses	6,863	11,288
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	1,475	26,342
Less Than 12 Months, Unrealized Losses	1	166
More Than 12 Months, Fair Value	54,656	54,900
More Than 12 Months, Unrealized Losses	798	1,924
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	1,260	28,322
Less Than 12 Months, Unrealized Losses	14	477
More Than 12 Months, Fair Value	15,040	21,560
More Than 12 Months, Unrealized Losses	153	534
Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	23,335	149,270
Less Than 12 Months, Unrealized Losses	273	4,483
More Than 12 Months, Fair Value	98,653	59,397
More Than 12 Months, Unrealized Losses	2,380	2,968
Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	19,724	82,594
Less Than 12 Months, Unrealized Losses	28	913
More Than 12 Months, Fair Value	210,423	181,379
More Than 12 Months, Unrealized Losses	$ 3,532	$ 5,862