Investments - Gross Unrecognized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses, Carrying Amount and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 476,605		$ 491,927
Held-to-Maturity, Allowance for Credit Losses	(204)	$ (220)	(239)
Held-to-Maturity, Carrying Amount	476,401		491,688
Held-to-Maturity, Gross Unrecognized Gains	1,198		983
Held-to-Maturity, Gross Unrecognized Losses	(21,986)		(24,527)
Held To Maturity Securities Fair Value	455,613		468,144
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	408,779		415,136
Held-to-Maturity, Allowance for Credit Losses	(41)	(40)	(40)
Held-to-Maturity, Carrying Amount	408,738		415,096
Held-to-Maturity, Gross Unrecognized Gains	1,165		922
Held-to-Maturity, Gross Unrecognized Losses	(18,023)		(20,074)
Held To Maturity Securities Fair Value	391,880		395,944
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	53,130		59,903
Held-to-Maturity, Allowance for Credit Losses	(161)	(178)	(196)
Held-to-Maturity, Carrying Amount	52,969		59,707
Held-to-Maturity, Gross Unrecognized Gains			1
Held-to-Maturity, Gross Unrecognized Losses	(3,381)		(3,857)
Held To Maturity Securities Fair Value	49,588		55,851
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	3,499		3,696
Held-to-Maturity, Carrying Amount	3,499		3,696
Held-to-Maturity, Gross Unrecognized Gains	15		33
Held-to-Maturity, Gross Unrecognized Losses	(150)		(153)
Held To Maturity Securities Fair Value	3,364		3,576
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	11,143		13,123
Held-to-Maturity, Carrying Amount	11,143		13,123
Held-to-Maturity, Gross Unrecognized Gains	18		25
Held-to-Maturity, Gross Unrecognized Losses	(431)		(442)
Held To Maturity Securities Fair Value	10,730		12,706
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	54		69
Held-to-Maturity, Allowance for Credit Losses	(2)	$ (2)	(3)
Held-to-Maturity, Carrying Amount	52		66
Held-to-Maturity, Gross Unrecognized Gains			2
Held-to-Maturity, Gross Unrecognized Losses	(1)		(1)
Held To Maturity Securities Fair Value	$ 51		$ 67