Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 545,662	$ 549,426
Held-to-Maturity, Allowance for Credit Losses	(290)	(195)
Held-to-Maturity, Carrying Amount	545,372	549,231
Held-to-Maturity, Gross Unrecognized Gains	7,524	27,032
Held-to-Maturity, Gross Unrecognized Losses	(7,471)	(413)
Held-to-Maturity, Fair Value	545,425	575,850
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	458,118	471,688
Held-to-Maturity, Allowance for Credit Losses	(45)	(48)
Held-to-Maturity, Carrying Amount	458,073	471,640
Held-to-Maturity, Gross Unrecognized Gains	7,160	25,175
Held-to-Maturity, Gross Unrecognized Losses	(5,936)	(263)
Held-to-Maturity, Fair Value	459,297	496,552
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	66,944	56,756
Held-to-Maturity, Allowance for Credit Losses	(241)	(143)
Held-to-Maturity, Carrying Amount	66,703	56,613
Held-to-Maturity, Gross Unrecognized Gains	137	1,344
Held-to-Maturity, Gross Unrecognized Losses	(1,371)	(114)
Held-to-Maturity, Fair Value	65,469	57,843
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	4,285	4,623
Held-to-Maturity, Carrying Amount	4,285	4,623
Held-to-Maturity, Gross Unrecognized Gains	167	377
Held-to-Maturity, Gross Unrecognized Losses	(5)
Held-to-Maturity, Fair Value	4,447	5,000
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	16,215	16,251
Held-to-Maturity, Carrying Amount	16,215	16,251
Held-to-Maturity, Gross Unrecognized Gains	56	132
Held-to-Maturity, Gross Unrecognized Losses	(159)	(36)
Held-to-Maturity, Fair Value	16,112	16,347
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	100	108
Held-to-Maturity, Allowance for Credit Losses	(4)	(4)
Held-to-Maturity, Carrying Amount	96	104
Held-to-Maturity, Gross Unrecognized Gains	4	4
Held-to-Maturity, Fair Value	$ 100	$ 108