Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	$ 5,066,262,000	$ 5,474,359,000	$ 6,215,036,000
Loans held for sale	19,220,000	11,965,000	26,445,000
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	483,776,000	482,549,000	486,358,000
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	1,692,964,000	1,898,611,000	2,244,604,000
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	2,472,941,000	2,616,566,000	2,964,904,000
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	314,387,000	374,869,000	414,825,000
IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	455,000	400,000	1,414,000
Liability Derivative Fair Value	0	0	0
Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	160,000	85,000	58,000
Liability Derivative Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	5,066,262,000	5,474,359,000	6,215,036,000
Loans held for sale	18,089,000	10,497,000	23,848,000
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	483,776,000	482,549,000	486,358,000
Fair Value, Measurements, Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	1,692,964,000	1,898,611,000	2,244,604,000
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	2,472,941,000	2,616,566,000	2,964,904,000
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	314,387,000	374,869,000	414,825,000
Fair Value, Measurements, Recurring [Member] | IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	455,000	400,000	1,414,000
Fair Value, Measurements, Recurring [Member] | Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Liability Derivative Fair Value	160,000	85,000	58,000
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	98,280,000	97,850,000	100,267,000
Fair Value, Measurements, Recurring [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	3,914,000	3,914,000	4,078,000
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	487,690,000	486,463,000	490,436,000
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:
Securities available-for-sale, Estimated Fair Value	483,776,000	482,549,000	486,358,000
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Securities available-for-sale, 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:
Securities available-for-sale, 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:
Securities available-for-sale, 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
Liability Derivative Fair Value	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	3,914,000	3,914,000	4,078,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	4,578,572,000	4,987,896,000	5,724,600,000
Loans held for sale	18,089,000	10,497,000	23,848,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, 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:
Securities available-for-sale, Estimated Fair Value	1,692,964,000	1,898,611,000	2,244,604,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	2,472,941,000	2,616,566,000	2,964,904,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	314,387,000	374,869,000	414,825,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	455,000	400,000	1,414,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Liability Derivative Fair Value	160,000	85,000	58,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	98,280,000	97,850,000	100,267,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	0		0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member]
Available-for-sale investment securities:
Securities available-for-sale, 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:
Securities available-for-sale, 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:
Securities available-for-sale, 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:
Securities available-for-sale, 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:
Securities available-for-sale, 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	0	0	0
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
Liability Derivative Fair Value	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Securities available-for-sale, Estimated Fair Value	$ 0	$ 0	$ 0