Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments, Not Previously Presented (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	$ 758,194	$ 402,828	$ 410,521
Federal Home Loan Bank stock	40,584	37,927	37,927
Commercial	1,381,216	1,336,820	1,294,015
Commercial real estate	1,165,766	1,255,883	1,291,054
Residential real estate	1,344,350	1,324,703	1,279,238
Consumer credit	930,343	906,855	930,070
FDIC indemnification asset	91,558	116,624	127,799	168,881
Accrued interest receivable	48,375	46,979	46,430
Noninterest-bearing demand deposits	1,975,153	2,007,770	1,943,525
Time deposits	1,108,217	1,281,281	1,361,087
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	247,410	264,060
Accrued interest receivable	109	43
Noninterest-bearing demand deposits	1,975,153	2,007,770
NOW, savings and money market deposits	4,125,037	3,989,902
Federal funds purchased	60,813	231,688
Repurchase agreements, short term	358,158	358,123
Fair Value, Inputs, Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Federal Home Loan Bank stock	40,584	37,927
Accrued interest receivable	20,104	20,701
Time deposits	1,121,495	1,308,111
Junior subordinated debenture	17,284	16,255
Repurchase agreements	52,938	53,422
Capital lease obligation	5,245	5,657
Accrued interest payable	2,118	3,308
Fair Value, Inputs, Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Commercial	1,436,377	1,424,103
Commercial real estate	1,302,316	1,475,066
Residential real estate	1,440,028	1,458,672
Consumer credit	1,020,179	1,030,990
FDIC indemnification asset	58,791	106,976
Accrued interest receivable	28,162	26,235
Federal Home Loan Bank advances	563,499	170,664
Standby letters of credit	365	357
Off-Balance Sheet Financial Instruments	2,156	2,305
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	180,020	188,263
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	40,488	58,919
States and Political Subdivisions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity	547,468	169,282
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	558,044	183,021
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 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Investment securities - held-to-maturity		2,998
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	247,410	264,060
Federal Home Loan Bank stock	40,584	37,927
Commercial	1,402,000	1,377,817
Commercial real estate	1,244,432	1,407,420
Residential real estate	1,371,379	1,356,922
Consumer credit	1,007,347	999,672
FDIC indemnification asset	91,558	116,624
Accrued interest receivable	48,375	46,979
Noninterest-bearing demand deposits	1,975,153	2,007,770
NOW, savings and money market deposits	4,125,037	3,989,902
Time deposits	1,108,217	1,281,281
Federal funds purchased	60,813	231,688
Repurchase agreements, short term	358,158	358,127
Junior subordinated debenture	28,000	28,000
Repurchase agreements	50,000	50,000
Federal Home Loan Bank advances	554,526	155,323
Capital lease obligation	4,170	4,211
Accrued interest payable	2,118	3,308
Standby letters of credit	365	357
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	171,472	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	39,254	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	547,468	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