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	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 487,354		$ 491,927
Held-to-Maturity, Allowance for Credit Losses	(188)	$ (204)	(239)
Held-to-Maturity, Carrying Amount	487,166		491,688
Held-to-Maturity, Gross Unrecognized Gains	131		983
Held-to-Maturity, Gross Unrecognized Losses	(35,278)		(24,527)
Held To Maturity Securities Fair Value	452,019		468,144
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	420,017		415,136
Held-to-Maturity, Allowance for Credit Losses	(37)	(41)	(40)
Held-to-Maturity, Carrying Amount	419,980		415,096
Held-to-Maturity, Gross Unrecognized Gains	113		922
Held-to-Maturity, Gross Unrecognized Losses	(30,814)		(20,074)
Held To Maturity Securities Fair Value	389,279		395,944
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	52,727		59,903
Held-to-Maturity, Allowance for Credit Losses	(150)	(161)	(196)
Held-to-Maturity, Carrying Amount	52,577		59,707
Held-to-Maturity, Gross Unrecognized Gains			1
Held-to-Maturity, Gross Unrecognized Losses	(3,562)		(3,857)
Held To Maturity Securities Fair Value	49,015		55,851
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	3,403		3,696
Held-to-Maturity, Carrying Amount	3,403		3,696
Held-to-Maturity, Gross Unrecognized Gains	4		33
Held-to-Maturity, Gross Unrecognized Losses	(253)		(153)
Held To Maturity Securities Fair Value	3,154		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	11,162		13,123
Held-to-Maturity, Carrying Amount	11,162		13,123
Held-to-Maturity, Gross Unrecognized Gains	14		25
Held-to-Maturity, Gross Unrecognized Losses	(649)		(442)
Held To Maturity Securities Fair Value	10,527		12,706
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	45		69
Held-to-Maturity, Allowance for Credit Losses	(1)	$ (2)	(3)
Held-to-Maturity, Carrying Amount	44		66
Held-to-Maturity, Gross Unrecognized Gains			2
Held-to-Maturity, Gross Unrecognized Losses			(1)
Held To Maturity Securities Fair Value	$ 44		$ 67