Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments, Not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, money market, and other interest-earning investments	$ 273,874	$ 290,432	$ 262,741	$ 255,519
Investment securities - held-to-maturity	525,718	684,063	695,139
Federal Home Loan Bank/Federal Reserve Bank stock	136,206	119,686	109,715
Commercial	2,962,895	2,717,269	2,001,621
Commercial real estate	4,451,772	4,354,552	3,259,998
Residential real estate	2,153,973	2,167,053	2,099,374
Consumer credit	1,726,989	1,879,247	1,871,047
Accrued interest receivable	81,290	87,102	79,830
Noninterest-bearing demand deposits	3,600,793	3,680,807	3,011,156
Time deposits	1,824,550	1,634,436	1,435,665
Federal funds purchased and interbank borrowings	175,044	335,033	227,029
Securities sold under agreements to repurchase	347,511	384,810	$ 298,094
Other borrowings	1,757,308	1,609,579
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, money market, and other interest-earning investments	273,874	290,432
Accrued interest receivable	13	16
Noninterest-bearing demand deposits	3,600,793	3,680,807
Checking, NOW, savings, and money market interest-bearing deposits	7,171,033	7,290,521
Federal funds purchased and interbank borrowings	175,044	335,033
Securities sold under agreements to repurchase	347,511	359,810
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued interest receivable	23,122	24,001
Time deposits	1,822,203	1,620,685
Securities sold under agreements to repurchase		25,133
Other borrowings	249,150	250,443
Accrued interest payable	7,510	7,029
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial	2,845,502	2,707,385
Commercial real estate	4,330,899	4,347,949
Residential real estate	2,117,999	2,210,951
Consumer credit	1,669,906	1,998,194
Accrued interest receivable	58,155	63,085
FHLB advances	1,747,022	1,607,189
Standby letters of credit	397	351
Commitments to extend credit	3,847	2,449
US Government Agencies Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	71,284
Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	139,594	6,903
Mortgage-Backed Securities - Agency [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	135,784	7,056
States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	312,554	677,160
States and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	317,529	720,647
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, money market, and other interest-earning investments	273,874	290,432
Federal Home Loan Bank/Federal Reserve Bank stock	136,206	119,686
Commercial	2,941,508	2,698,023
Commercial real estate	4,429,274	4,333,116
Residential real estate	2,152,106	2,165,290
Consumer credit	1,719,081	1,871,311
Accrued interest receivable	81,290	87,102
Noninterest-bearing demand deposits	3,600,793	3,680,807
Checking, NOW, savings, and money market interest-bearing deposits	7,171,033	7,290,521
Time deposits	1,824,550	1,634,436
Federal funds purchased and interbank borrowings	175,044	335,033
Securities sold under agreements to repurchase	347,511	384,810
FHLB advances	1,757,308	1,609,579
Other borrowings	250,241	248,782
Accrued interest payable	7,510	7,029
Standby letters of credit	397	351
Carrying Value [Member] | US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	73,570
Carrying Value [Member] | Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	139,594	6,903
Carrying Value [Member] | States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	$ 312,554	$ 677,160