Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments, Not Previously Presented (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	$ 836,038	$ 844,054	$ 779,294
Federal Home Loan Bank/Federal Reserve stock	75,068	71,175	61,882
Loans held for sale	186,200
Commercial	1,668,275	1,629,600	1,367,486
Commercial real estate	1,813,579	1,711,110	1,156,593
Residential real estate	1,625,354	1,519,156	1,356,233
Consumer credit	1,408,491	1,310,627	997,808
FDIC indemnification asset	20,024	20,603	65,699	88,513
Accrued interest receivable	62,503	60,966	48,764
Noninterest-bearing demand deposits	2,553,801	2,427,748	2,047,664
Time deposits	1,134,041	1,089,018	960,804
Repurchase agreements	463,007	551,309	410,128
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	215,313	239,963
Accrued interest receivable	50	29
Noninterest-bearing demand deposits	2,553,801	2,427,748
NOW, savings and money market deposits	5,239,678	4,973,898
Federal funds purchased	93,492	195,188
Repurchase agreements	369,513	356,120
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Federal Home Loan Bank/Federal Reserve stock	75,068	71,175
Loans held for sale	186,169	197,928
Accrued interest receivable	21,157	21,633
Time deposits	1,139,301	1,092,969
Senior unsecured bank notes	173,378	179,792
Junior subordinated debentures	32,667	32,754
Repurchase agreements	52,067	51,994
Capital lease obligation	5,709	5,515
Accrued interest payable	2,878	4,564
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial	1,708,002	1,646,144
Commercial real estate	1,938,556	1,744,126
Residential real estate	1,936,855	1,615,588
Consumer credit	1,477,229	1,380,835
FDIC indemnification asset	10,639	11,358
Accrued interest receivable	41,296	39,304
Federal Home Loan Bank advances	608,756	658,506
Standby letters of credit	355	358
Commitments to extend credit	3,292	2,030
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	172,636	173,486
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	22,470	24,574
States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	648,147	653,199
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	704,547	705,875
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	215,313	239,963
Federal Home Loan Bank/Federal Reserve stock	75,068	71,175
Loans held for sale	186,169	197,928
Commercial	1,657,401	1,626,097
Commercial real estate	1,836,271	1,734,559
Residential real estate	1,642,683	1,537,448
Consumer credit	1,467,306	1,372,248
FDIC indemnification asset	20,024	20,603
Accrued interest receivable	62,503	60,966
Noninterest-bearing demand deposits	2,553,801	2,427,748
NOW, savings and money market deposits	5,239,678	4,973,898
Time deposits	1,134,041	1,089,018
Federal funds purchased	93,492	195,188
Repurchase agreements	369,515	356,121
Senior unsecured bank notes	175,000	175,000
Junior subordinated debentures	45,000	45,000
Repurchase agreements	50,000	50,000
Federal Home Loan Bank advances	599,874	649,987
Capital lease obligation	4,083	4,099
Accrued interest payable	2,878	4,564
Standby letters of credit	355	358
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	166,343	167,207
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	21,548	23,648
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	$ 648,147	$ 653,199