Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Available-for-sale investment securities:
Estimated Fair Value	$ 4,573,024,000	$ 4,732,762,000	$ 5,066,262,000
Loans held for sale	19,668,000	14,253,000	19,220,000
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	341,976,000	482,234,000	483,776,000
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,445,373,000	1,497,157,000	1,692,964,000
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,316,085,000	2,364,092,000	2,472,941,000
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	350,022,000	284,324,000	314,387,000
IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	518,000	427,000	455,000
Forward mortgage-backed securities trades asset (liability)	0	0	0
Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	0	160,000
Forward mortgage-backed securities trades asset (liability)	4,000	288,000	0
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,573,024,000	4,732,762,000	5,066,262,000
Loans held for sale	19,393,000	11,077,000	18,089,000
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	341,976,000	482,234,000	483,776,000
Fair Value, Measurements, Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,445,373,000	1,497,157,000	1,692,964,000
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,316,085,000	2,364,092,000	2,472,941,000
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	350,022,000	284,324,000	314,387,000
Fair Value, Measurements, Recurring [Member] | IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	518,000	427,000	455,000
Fair Value, Measurements, Recurring [Member] | Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades asset (liability)	(4,000)	(288,000)	160,000
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	99,627,000	100,471,000	98,280,000
Fair Value, Measurements, Recurring [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	19,941,000	4,484,000	3,914,000
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	361,917,000	486,718,000	487,690,000
Loans held for sale	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	341,976,000	482,234,000	483,776,000
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value			0
Forward mortgage-backed securities trades asset (liability)	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	19,941,000	4,484,000	3,914,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,211,107,000	4,246,044,000	4,578,572,000
Loans held for sale	19,393,000	11,077,000	18,089,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,445,373,000	1,497,157,000	1,692,964,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,316,085,000	2,364,092,000	2,472,941,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	350,022,000	284,324,000	314,387,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades asset (liability)	(4,000)	(288,000)	160,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	99,627,000	100,471,000	98,280,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Loans held for sale	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	518,000	427,000	455,000
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value			0
Forward mortgage-backed securities trades asset (liability)	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	$ 0	$ 0	$ 0