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	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 480,362	$ 491,927
Held-to-Maturity, Allowance for Credit Losses	(220)	(239)
Held-to-Maturity, Carrying Amount	480,142	491,688
Held-to-Maturity, Gross Unrecognized Gains	2,022	983
Held-to-Maturity, Gross Unrecognized Losses	(18,350)	(24,527)
Held To Maturity Securities Fair Value	463,814	468,144
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	412,038	415,136
Held-to-Maturity, Allowance for Credit Losses	(40)	(40)
Held-to-Maturity, Carrying Amount	411,998	415,096
Held-to-Maturity, Gross Unrecognized Gains	1,965	922
Held-to-Maturity, Gross Unrecognized Losses	(14,777)	(20,074)
Held To Maturity Securities Fair Value	399,186	395,944
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	53,528	59,903
Held-to-Maturity, Allowance for Credit Losses	(178)	(196)
Held-to-Maturity, Carrying Amount	53,350	59,707
Held-to-Maturity, Gross Unrecognized Gains	2	1
Held-to-Maturity, Gross Unrecognized Losses	(3,170)	(3,857)
Held To Maturity Securities Fair Value	50,182	55,851
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	3,606	3,696
Held-to-Maturity, Carrying Amount	3,606	3,696
Held-to-Maturity, Gross Unrecognized Gains	30	33
Held-to-Maturity, Gross Unrecognized Losses	(126)	(153)
Held To Maturity Securities Fair Value	3,510	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	11,125	13,123
Held-to-Maturity, Carrying Amount	11,125	13,123
Held-to-Maturity, Gross Unrecognized Gains	25	25
Held-to-Maturity, Gross Unrecognized Losses	(276)	(442)
Held To Maturity Securities Fair Value	10,874	12,706
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	65	69
Held-to-Maturity, Allowance for Credit Losses	(2)	(3)
Held-to-Maturity, Carrying Amount	63	66
Held-to-Maturity, Gross Unrecognized Gains		2
Held-to-Maturity, Gross Unrecognized Losses	(1)	(1)
Held To Maturity Securities Fair Value	$ 62	$ 67