Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments, Not Previously Presented (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, money market, and other interest-earning investments	$ 290,432	$ 255,519	$ 219,818	$ 239,963
Investment securities - held-to-maturity	684,063	745,090
Federal Home Loan Bank/Federal Reserve Bank stock	119,686	101,716
Accrued interest receivable	87,102	81,381
Noninterest-bearing demand deposits	3,680,807	3,016,093
Time deposits	1,634,436	1,468,108
Federal funds purchased and interbank borrowings	335,033	213,003
Securities sold under agreements to repurchase	384,810	367,052
Other borrowings	1,609,579	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, money market, and other interest-earning investments	290,432	255,519
Accrued interest receivable	16	16
Noninterest-bearing demand deposits	3,680,807	3,016,093
NOW, savings, and money market deposits	7,290,521	6,259,052
Federal funds purchased and interbank borrowings	335,033	213,003
Securities sold under agreements to repurchase	359,810	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	24,001	22,880
Time deposits	1,620,685	1,460,778
Securities sold under agreements to repurchase	25,133	50,612
Other borrowings	250,443	217,647
Accrued interest payable	7,029	5,979
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial	2,707,385	1,971,296
Commercial real estate	4,347,949	3,400,365
Residential real estate	2,210,951	2,228,542
Consumer credit	1,998,194	1,974,180
Accrued interest receivable	63,085	58,485
Federal Home Loan Bank advances	1,607,189	1,360,599
Standby letters of credit	351	315
Commitments to extend credit	2,449	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		40,131
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		40,558
Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	6,903	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	7,056	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	677,160	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	720,647	732,674
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	290,432	255,519
Federal Home Loan Bank/Federal Reserve Bank stock	119,686	101,716
Commercial	2,698,023	1,895,618
Commercial real estate	4,333,116	3,112,680
Residential real estate	2,165,290	2,085,887
Consumer credit	1,871,311	1,866,519
Accrued interest receivable	87,102	81,381
Noninterest-bearing demand deposits	3,680,807	3,016,093
NOW, savings, and money market deposits	7,290,521	6,259,052
Time deposits	1,634,436	1,468,108
Federal funds purchased and interbank borrowings	335,033	213,003
Securities sold under agreements to repurchase	384,810	367,052
Federal Home Loan Bank advances	1,609,579	1,353,092
Other borrowings	248,782	218,939
Accrued interest payable	7,029	5,979
Standby letters of credit	351	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		40,131
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	6,903	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	$ 677,160	$ 694,319