Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments, Not Previously Presented (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	$ 852,904	$ 762,734	$ 401,066
Federal Home Loan Bank stock	42,776	40,584	40,584
Commercial	1,498,833	1,373,415	1,397,882
Commercial real estate	1,354,700	1,160,890	1,197,997
Residential real estate	1,425,179	1,359,569	1,399,688
Consumer credit	1,089,008	971,258	892,078
FDIC indemnification asset	51,431	88,513	100,391	116,624
Accrued interest receivable	54,630	50,205	48,516
Noninterest-bearing demand deposits	2,129,705	2,026,490	1,881,440
Time deposits	984,929	1,017,975	1,122,003
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	236,693	206,723
Accrued interest receivable	42	42
Noninterest-bearing demand deposits	2,129,705	2,026,490
NOW, savings and money market deposits	4,440,369	4,166,438
Federal funds purchased	150,188	115,103
Repurchase agreements	317,389	347,226
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Federal Home Loan Bank stock	42,776	40,584
Accrued interest receivable	21,930	20,708
Time deposits	993,298	1,028,043
Junior subordinated debenture	33,206	17,605
Repurchase agreements	52,481	52,633
Capital lease obligation	5,631	5,245
Accrued interest payable	1,764	1,877
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial	1,540,735	1,414,184
Commercial real estate	1,452,514	1,273,070
Residential real estate	1,541,606	1,475,710
Consumer credit	1,164,479	1,058,021
FDIC indemnification asset	22,332	55,368
Accrued interest receivable	32,658	29,455
Federal Home Loan Bank advances	816,743	485,759
Standby letters of credit	365	380
Commitments to extend credit	2,111	1,648
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	177,117	178,370
Mortgage-Backed 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	30,264	36,348
States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	655,038	556,670
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	691,626	566,040
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	236,693	206,723
Federal Home Loan Bank stock	42,776	40,584
Commercial	1,501,651	1,386,185
Commercial real estate	1,387,950	1,220,417
Residential real estate	1,446,598	1,384,183
Consumer credit	1,156,517	1,045,034
FDIC indemnification asset	51,431	88,513
Accrued interest receivable	54,630	50,205
Noninterest-bearing demand deposits	2,129,705	2,026,490
NOW, savings and money market deposits	4,440,369	4,166,438
Time deposits	984,929	1,017,975
Federal funds purchased	150,188	115,103
Repurchase agreements	317,390	347,229
Junior subordinated debenture	45,000	28,000
Repurchase agreements	50,000	50,000
Federal Home Loan Bank advances	807,379	447,856
Capital lease obligation	4,128	4,157
Accrued interest payable	1,764	1,877
Standby letters of credit	365	380
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	168,936	170,621
Carrying Value [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities - held-to-maturity	28,930	35,443
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	$ 655,038	$ 556,670