v3.26.1
Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
Mar. 31, 2026
Dec. 31, 2025
Available-for-sale investment securities:    
Estimated Fair Value $ 5,668,792,000 $ 5,514,113,000
Loans held for sale 22,984,000 29,992,000
U.S. Treasury Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 19,962,000 60,813,000
Obligations of State and Political Subdivisions [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 1,729,722,000 1,751,373,000
Residential Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 2,683,620,000 2,611,410,000
Commercial Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 1,138,518,000 989,238,000
IRLCs [Member]    
Available-for-sale investment securities:    
Asset Derivative Fair Value 723,000 579,000
Forward mortgage-backed securities trades 0 0
Forward Mortgage-Backed Securities Trades [Member]    
Available-for-sale investment securities:    
Asset Derivative Fair Value 515,000 0
Forward mortgage-backed securities trades 0 (132,000)
Fair Value, Measurements, Recurring [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 5,668,792,000 5,514,113,000
Loans held for sale 18,136,000 25,431,000
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 19,962,000 60,813,000
Fair Value, Measurements, Recurring [Member] | Obligations of State and Political Subdivisions [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 1,729,722,000 1,751,373,000
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 68,429,000 78,709,000
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 2,683,620,000 2,611,410,000
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 1,138,518,000 989,238,000
Fair Value, Measurements, Recurring [Member] | IRLCs [Member]    
Available-for-sale investment securities:    
Asset Derivative Fair Value 723,000 579,000
Fair Value, Measurements, Recurring [Member] | Forward Mortgage-Backed Securities Trades [Member]    
Available-for-sale investment securities:    
Forward mortgage-backed securities trades (515,000) 132,000
Fair Value, Measurements, Recurring [Member] | Collateralized Securities, Other [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 28,541,000 22,570,000
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 48,503,000 83,383,000
Loans held for sale 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | U.S. Treasury Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 19,962,000 60,813,000
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
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Corporate Bonds [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 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
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | IRLCs [Member]    
Available-for-sale investment securities:    
Asset Derivative Fair Value 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Forward Mortgage-Backed Securities Trades [Member]    
Available-for-sale investment securities:    
Forward mortgage-backed securities trades 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Collateralized Securities, Other [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 28,541,000 22,570,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 5,620,289,000 5,430,730,000
Loans held for sale 18,136,000 25,431,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | U.S. Treasury Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 0 0
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Obligations of State and Political Subdivisions [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 1,729,722,000 1,751,373,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Corporate Bonds [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 68,429,000 78,709,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Residential Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 2,683,620,000 2,611,410,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 1,138,518,000 989,238,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | IRLCs [Member]    
Available-for-sale investment securities:    
Asset Derivative Fair Value 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 (515,000) 132,000
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Collateralized Securities, Other [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 0 0
Loans held for sale 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
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Corporate Bonds [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 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
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]    
Available-for-sale investment securities:    
Estimated Fair Value 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | IRLCs [Member]    
Available-for-sale investment securities:    
Asset Derivative Fair Value 723,000 579,000
Fair Value, Measurements, Recurring [Member] | Level 3 Inputs [Member] | Forward Mortgage-Backed Securities Trades [Member]    
Available-for-sale investment securities:    
Forward mortgage-backed securities trades 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