Investments - Summary of Fixed Maturities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 13,971	$ 286,528
Less Than 12 Months, Unrealized Losses	78	6,039
More Than 12 Months, Fair Value	200,685	317,236
More Than 12 Months, Unrealized Losses	2,002	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		26,342
Less Than 12 Months, Unrealized Losses		166
More Than 12 Months, Fair Value	25,618	54,900
More Than 12 Months, Unrealized Losses	111	1,924
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	3,740	28,322
Less Than 12 Months, Unrealized Losses	15	477
More Than 12 Months, Fair Value	4,864	21,560
More Than 12 Months, Unrealized Losses	8	534
Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	6,538	149,270
Less Than 12 Months, Unrealized Losses	58	4,483
More Than 12 Months, Fair Value	40,055	59,397
More Than 12 Months, Unrealized Losses	624	2,968
Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	3,693	82,594
Less Than 12 Months, Unrealized Losses	5	913
More Than 12 Months, Fair Value	130,148	181,379
More Than 12 Months, Unrealized Losses	$ 1,259	$ 5,862