Investments - Summary of Fixed Maturities with Unrealized Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 190,500	$ 67,740
Less Than 12 Months, Unrealized Losses	7,223	629
More Than 12 Months, Fair Value	10,903	8,466
More Than 12 Months, Unrealized Losses	406	277
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	33,349	29,144
Less Than 12 Months, Unrealized Losses	1,939	345
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	88,703	9,362
Less Than 12 Months, Unrealized Losses	3,255	154
More Than 12 Months, Fair Value	2,094
More Than 12 Months, Unrealized Losses	205
Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	46,939	26,143
Less Than 12 Months, Unrealized Losses	1,403	115
More Than 12 Months, Fair Value	8,289	8,230
More Than 12 Months, Unrealized Losses	196	276
Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	21,509	3,091
Less Than 12 Months, Unrealized Losses	626	15
More Than 12 Months, Fair Value	520	236
More Than 12 Months, Unrealized Losses	$ 5	$ 1