Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 549,426	$ 585,404
Held-to-Maturity, Allowance for Credit Losses	(195)	(274)
Held-to-Maturity, Carrying Amount	549,231	585,130
Held-to-Maturity, Gross Unrecognized Gains	27,032	36,524
Held-to-Maturity, Gross Unrecognized Losses	(413)
Held-to-Maturity, Fair Value	575,850	621,654
Fixed maturity securities, held-to-maturity, allowance for credit losses	195	274
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	471,688	494,374
Held-to-Maturity, Allowance for Credit Losses	(48)	(42)
Held-to-Maturity, Carrying Amount	471,640	494,332
Held-to-Maturity, Gross Unrecognized Gains	25,175	32,489
Held-to-Maturity, Gross Unrecognized Losses	(263)	0
Held-to-Maturity, Fair Value	496,552	526,821
Fixed maturity securities, held-to-maturity, allowance for credit losses	48	42
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	56,756	69,981
Held-to-Maturity, Allowance for Credit Losses	(143)	(225)
Held-to-Maturity, Carrying Amount	56,613	69,756
Held-to-Maturity, Gross Unrecognized Gains	1,344	3,144
Held-to-Maturity, Gross Unrecognized Losses	(114)
Held-to-Maturity, Fair Value	57,843	72,900
Fixed maturity securities, held-to-maturity, allowance for credit losses	143	225
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	4,623	7,261
Held-to-Maturity, Carrying Amount	4,623	7,261
Held-to-Maturity, Gross Unrecognized Gains	377	645
Held-to-Maturity, Fair Value	5,000	7,906
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	16,251	13,626
Held-to-Maturity, Carrying Amount	16,251	13,626
Held-to-Maturity, Gross Unrecognized Gains	132	239
Held-to-Maturity, Gross Unrecognized Losses	(36)	0
Held-to-Maturity, Fair Value	16,347	13,865
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	108	162
Held-to-Maturity, Allowance for Credit Losses	(4)	(7)
Held-to-Maturity, Carrying Amount	104	155
Held-to-Maturity, Gross Unrecognized Gains	4	7
Held-to-Maturity, Fair Value	108	162
Fixed maturity securities, held-to-maturity, allowance for credit losses	$ 4	$ 7