Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Available-for-sale investment securities:
Available-for-sale securities	$ 4,658,526,000	$ 4,732,762,000	$ 5,298,557,000
Loans held for sale	16,109,000	14,253,000	11,996,000
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	432,168,000	482,234,000	488,242,000
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale securities	1,454,035,000	1,497,157,000	1,773,612,000
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	2,299,277,000	2,364,092,000	2,583,170,000
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	368,906,000	284,324,000	349,936,000
IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	803,000	427,000	827,000
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	0
Forward mortgage-backed securities trades asset (liability)	91,000	288,000	334,000
Level 2 Inputs [Member] | Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades asset (liability)	(91,000)	(288,000)	(334,000)
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Available-for-sale securities	4,658,526,000	4,732,762,000	5,298,557,000
Loans held for sale	15,080,000	11,077,000	11,766,000
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	432,168,000	482,234,000	488,242,000
Fair Value, Measurements, Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale securities	1,454,035,000	1,497,157,000	1,773,612,000
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	2,299,277,000	2,364,092,000	2,583,170,000
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	368,906,000	284,324,000	349,936,000
Fair Value, Measurements, Recurring [Member] | IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	803,000	427,000	827,000
Fair Value, Measurements, Recurring [Member] | Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades asset (liability)	(91,000)	(288,000)	(334,000)
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale securities	99,717,000	100,471,000	99,620,000
Fair Value, Measurements, Recurring [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Available-for-sale securities	4,423,000	4,484,000	3,977,000
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale securities	436,591,000	486,718,000	492,219,000
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:
Available-for-sale securities	432,168,000	482,234,000	488,242,000
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	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:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Available-for-sale securities	4,423,000	4,484,000	3,977,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale securities	4,221,935,000	4,246,044,000	4,806,338,000
Loans held for sale	15,080,000	11,077,000	11,766,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale securities	1,454,035,000	1,497,157,000	1,773,612,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	2,299,277,000	2,364,092,000	2,583,170,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	368,906,000	284,324,000	349,936,000
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)	(91,000)	(288,000)	(334,000)
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale securities	99,717,000	100,471,000	99,620,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale securities	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:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	803,000	427,000	827,000
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:
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Available-for-sale securities	$ 0	$ 0	$ 0