Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments, Not Previously Presented (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, federal funds sold, and money market investments	$ 217,035	$ 255,519	$ 175,558	$ 219,818
Investment securities - held-to-maturity	741,448	745,090	869,012
Federal Home Loan Bank/Federal Reserve Bank stock	107,501	101,716	86,146
Commercial	1,910,536	1,917,099	1,792,988
Commercial real estate	3,222,865	3,130,853	1,922,754
Residential real estate	2,112,262	2,087,530	1,649,996
Consumer credit	1,886,110	1,875,030	1,641,336
Accrued interest receivable	76,674	81,381	68,641
Noninterest-bearing demand deposits	3,024,111	3,016,093	2,491,767
Time deposits	1,466,718	1,468,108	1,085,847
Federal funds purchased and interbank borrowings	61,016	213,003	165,320
Securities sold under agreements to repurchase	345,550	367,052	$ 379,060
Other borrowings	1,441,030	1,353,092
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, federal funds sold, and money market investments	217,035	255,519
Accrued interest receivable	50	16
Noninterest-bearing demand deposits	3,024,111	3,016,093
NOW, savings, and money market deposits	6,330,523	6,259,052
Federal funds purchased and interbank borrowings	61,016	213,003
Securities sold under agreements to repurchase	320,550	317,052
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued interest receivable	19,916	22,880
Time deposits	1,457,966	1,460,778
Securities sold under agreements to repurchase	25,360	50,612
Other borrowings	217,808	217,647
Accrued interest payable	4,004	5,979
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial	1,967,837	1,971,296
Commercial real estate	3,499,147	3,400,365
Residential real estate	2,248,048	2,228,542
Consumer credit	1,989,675	1,974,180
Accrued interest receivable	56,708	58,485
Federal Home Loan Bank advances	1,448,233	1,360,599
Standby letters of credit	349	315
Commitments to extend credit	3,363	2,527
U.S. Government-Sponsored Entities and Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	40,021	40,131
U.S. Government-Sponsored Entities and Agencies [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	40,110	40,558
Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	9,566	10,640
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	9,854	10,940
States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	691,861	694,319
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	734,942	732,674
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, federal funds sold, and money market investments	217,035	255,519
Federal Home Loan Bank/Federal Reserve Bank stock	107,501	101,716
Commercial	1,888,428	1,895,618
Commercial real estate	3,204,912	3,112,680
Residential real estate	2,110,526	2,085,887
Consumer credit	1,878,073	1,866,519
Accrued interest receivable	76,674	81,381
Noninterest-bearing demand deposits	3,024,111	3,016,093
NOW, savings, and money market deposits	6,330,523	6,259,052
Time deposits	1,466,718	1,468,108
Federal funds purchased and interbank borrowings	61,016	213,003
Securities sold under agreements to repurchase	345,550	367,052
Federal Home Loan Bank advances	1,441,030	1,353,092
Other borrowings	219,021	218,939
Accrued interest payable	4,004	5,979
Standby letters of credit	349	315
Carrying Value [Member] | U.S. Government-Sponsored Entities and Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	40,021	40,131
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	9,566	10,640
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	$ 691,861	$ 694,319