Allowance for Credit Losses - Summary of Allocation of Allowance for Loan Losses and Related Loans by Loan Portfolio Segment Disaggregated Based on Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Allowance for loan losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 28,150	$ 31,013
Collectively evaluated for impairment	86,514	89,695
Loans acquired with discounts related to credit quality	7,180	9,492
Total	121,844	130,200
Allowance for loan losses [Member] | Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	9,759	12,088
Collectively evaluated for impairment	45,461	44,877
Loans acquired with discounts related to credit quality	512	7,405
Total	55,732	64,370
Allowance for loan losses [Member] | Commercial real estate loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	14,478	16,581
Collectively evaluated for impairment	23,285	25,463
Loans acquired with discounts related to credit quality	6,432	2,025
Total	44,195	44,069
Allowance for loan losses [Member] | Residential mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	3,897	2,329
Collectively evaluated for impairment	5,201	7,032
Loans acquired with discounts related to credit quality	233	62
Total	9,331	9,423
Allowance for loan losses [Member] | Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	16	15
Collectively evaluated for impairment	5,441	5,527
Loans acquired with discounts related to credit quality	3
Total	5,460	5,542
Allowance for loan losses [Member] | Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment
Collectively evaluated for impairment	7,126	6,796
Loans acquired with discounts related to credit quality
Total	7,126	6,796
Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	208,667	206,734
Collectively evaluated for impairment	9,531,804	9,648,401
Loans acquired with discounts related to credit quality	1,071,028	1,167,664
Total	10,811,499	11,022,799
Loans [Member] | Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	48,633	49,697
Collectively evaluated for impairment	1,762,928	1,783,046
Loans acquired with discounts related to credit quality	268,644	298,600
Total	2,080,205	2,131,343
Loans [Member] | Commercial real estate loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	130,966	131,216
Collectively evaluated for impairment	4,039,801	4,040,723
Loans acquired with discounts related to credit quality	734,817	793,406
Total	4,905,584	4,965,345
Loans [Member] | Residential mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	27,824	24,891
Collectively evaluated for impairment	2,308,997	2,420,736
Loans acquired with discounts related to credit quality	24,750	26,461
Total	2,361,571	2,472,088
Loans [Member] | Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,244	930
Collectively evaluated for impairment	1,420,078	1,403,896
Loans acquired with discounts related to credit quality	42,817	49,197
Total	1,464,139	1,454,023
Loans [Member] | Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment
Collectively evaluated for impairment
Loans acquired with discounts related to credit quality
Total