Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments, Not Previously Presented (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	$ 401,066	$ 402,828	$ 463,935
Federal Home Loan Bank stock	40,584	37,927	30,835
Commercial	1,397,882	1,336,820	1,205,532
Commercial real estate	1,197,997	1,255,883	1,042,581
Residential real estate	1,399,688	1,324,703	1,122,800
Consumer credit	892,078	906,855	855,386
FDIC indemnification asset	100,391	116,624	136,994	168,881
Accrued interest receivable	48,516	46,979	44,163
Noninterest-bearing demand deposits	1,881,440	2,007,770	1,847,904
Time deposits	1,122,003	1,281,281	1,251,831
Fair Value, Inputs, Level 1 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Cash, due from banks, federal funds sold and money market investments	216,825	264,060
Federal Home Loan Bank stock
Commercial
Commercial real estate
Residential real estate
Consumer credit
FDIC indemnification asset
Accrued interest receivable	41	43
Noninterest-bearing demand deposits	1,881,440	2,007,770
NOW, savings and money market deposits	3,836,650	3,989,902
Time deposits
Federal funds purchased	105,190	231,688
Repurchase agreements, short term	425,190	358,123
Junior subordinated debenture
Repurchase agreements
Federal Home Loan Bank advances
Capital lease obligation
Accrued interest payable
Standby letters of credit
Off-Balance Sheet Financial Instruments
Fair Value, Inputs, Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Cash, due from banks, federal funds sold and money market investments
Federal Home Loan Bank stock	40,584	37,927
Commercial
Commercial real estate
Residential real estate
Consumer credit
FDIC indemnification asset
Accrued interest receivable	21,589	20,701
Noninterest-bearing demand deposits
NOW, savings and money market deposits
Time deposits	1,137,262	1,308,111
Federal funds purchased
Repurchase agreements, short term
Junior subordinated debenture	16,855	16,255
Repurchase agreements	52,916	53,422
Federal Home Loan Bank advances
Capital lease obligation	5,201	5,657
Accrued interest payable	2,854	3,308
Standby letters of credit
Off-Balance Sheet Financial Instruments
Fair Value, Inputs, Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Cash, due from banks, federal funds sold and money market investments
Federal Home Loan Bank stock
Commercial	1,457,421	1,424,103
Commercial real estate	1,360,091	1,475,066
Residential real estate	1,542,833	1,458,672
Consumer credit	996,343	1,030,990
FDIC indemnification asset	70,630	106,976
Accrued interest receivable	26,886	26,235
Noninterest-bearing demand deposits
NOW, savings and money market deposits
Time deposits
Federal funds purchased
Repurchase agreements, short term
Junior subordinated debenture
Repurchase agreements
Federal Home Loan Bank advances	812,251	170,664
Capital lease obligation
Accrued interest payable
Standby letters of credit	314	357
Off-Balance Sheet Financial Instruments	1,563	2,305
U.S. Government-Sponsored Entities and Agencies [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity
U.S. Government-Sponsored Entities and Agencies [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	181,224	188,263
U.S. Government-Sponsored Entities and Agencies [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity
Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	44,286	56,612
Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity
Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	45,985	58,919
Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity
States and Political Subdivisions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	184,481	169,282
States and Political Subdivisions [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity
States and Political Subdivisions [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	192,117	183,021
States and Political Subdivisions [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity
Other Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity		2,998
Other Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity
Other Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity		2,998
Other Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity
Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Cash, due from banks, federal funds sold and money market investments	216,825	264,060
Federal Home Loan Bank stock	40,584	37,927
Commercial	1,422,119	1,377,817
Commercial real estate	1,302,085	1,407,420
Residential real estate	1,428,853	1,356,922
Consumer credit	973,847	999,672
FDIC indemnification asset	100,391	116,624
Accrued interest receivable	48,516	46,979
Noninterest-bearing demand deposits	1,881,440	2,007,770
NOW, savings and money market deposits	3,836,650	3,989,902
Time deposits	1,122,003	1,281,281
Federal funds purchased	105,190	231,688
Repurchase agreements, short term	425,191	358,127
Junior subordinated debenture	28,000	28,000
Repurchase agreements	50,000	50,000
Federal Home Loan Bank advances	804,881	155,323
Capital lease obligation	4,184	4,211
Accrued interest payable	2,854	3,308
Standby letters of credit	314	357
Off-Balance Sheet Financial Instruments	0	0
Reported Value Measurement [Member] | U.S. Government-Sponsored Entities and Agencies [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	172,299	173,936
Reported Value Measurement [Member] | Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	44,286	56,612
Reported Value Measurement [Member] | States and Political Subdivisions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	184,481	169,282
Reported Value Measurement [Member] | Other Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity		$ 2,998