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, 2022	Jun. 30, 2022	Dec. 31, 2021
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 514,865		$ 549,426
Held-to-Maturity, Allowance for Credit Losses	(255)	$ (276)	(195)
Held-to-Maturity, Carrying Amount	514,610		549,231
Held-to-Maturity, Gross Unrecognized Gains	329		27,032
Held-to-Maturity, Gross Unrecognized Losses	(35,028)		(413)
Held To Maturity Securities Fair Value	479,911		575,850
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	434,938		471,688
Held-to-Maturity, Allowance for Credit Losses	(44)	(44)	(48)
Held-to-Maturity, Carrying Amount	434,894		471,640
Held-to-Maturity, Gross Unrecognized Gains	265		25,175
Held-to-Maturity, Gross Unrecognized Losses	(30,096)		(263)
Held To Maturity Securities Fair Value	405,063		496,552
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	62,390		56,756
Held-to-Maturity, Allowance for Credit Losses	(208)	(228)	(143)
Held-to-Maturity, Carrying Amount	62,182		56,613
Held-to-Maturity, Gross Unrecognized Gains	2		1,344
Held-to-Maturity, Gross Unrecognized Losses	(4,242)		(114)
Held To Maturity Securities Fair Value	57,942		57,843
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	3,812		4,623
Held-to-Maturity, Carrying Amount	3,812		4,623
Held-to-Maturity, Gross Unrecognized Gains	32		377
Held-to-Maturity, Gross Unrecognized Losses	(185)
Held To Maturity Securities Fair Value	3,659		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	13,647		16,251
Held-to-Maturity, Carrying Amount	13,647		16,251
Held-to-Maturity, Gross Unrecognized Gains	27		132
Held-to-Maturity, Gross Unrecognized Losses	(504)		(36)
Held To Maturity Securities Fair Value	13,170		16,347
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	78		108
Held-to-Maturity, Allowance for Credit Losses	(3)	$ (4)	(4)
Held-to-Maturity, Carrying Amount	75		104
Held-to-Maturity, Gross Unrecognized Gains	3		4
Held-to-Maturity, Gross Unrecognized Losses	(1)
Held To Maturity Securities Fair Value	$ 77		$ 108