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	Jun. 30, 2013	Dec. 31, 2012
Allowance for loan losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 28,740	$ 31,013
Collectively evaluated for impairment	81,634	89,695
Loans acquired with discounts related to credit quality	7,070	9,492
Total	117,444	130,200
Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	216,291	206,734
Collectively evaluated for impairment	9,715,329	9,648,401
Loans acquired with discounts related to credit quality	951,405	1,167,664
Total	10,883,025	11,022,799
Commercial and industrial [Member] | Allowance for loan losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	11,420	12,088
Collectively evaluated for impairment	41,800	44,877
Loans acquired with discounts related to credit quality	512	7,405
Total	53,732	64,370
Commercial and industrial [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	57,595	49,697
Collectively evaluated for impairment	1,743,223	1,783,046
Loans acquired with discounts related to credit quality	220,576	298,600
Total	2,021,394	2,131,343
Commercial real estate loans [Member] | Allowance for loan losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	14,632	16,581
Collectively evaluated for impairment	22,115	25,463
Loans acquired with discounts related to credit quality	6,432	2,025
Total	43,179	44,069
Commercial real estate loans [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	130,631	131,216
Collectively evaluated for impairment	4,166,945	4,040,723
Loans acquired with discounts related to credit quality	665,220	793,406
Total	4,962,796	4,965,345
Residential mortgage [Member] | Allowance for loan losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	2,677	2,329
Collectively evaluated for impairment	5,721	7,032
Loans acquired with discounts related to credit quality	123	62
Total	8,521	9,423
Residential mortgage [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	26,879	24,891
Collectively evaluated for impairment	2,371,005	2,420,736
Loans acquired with discounts related to credit quality	24,004	26,461
Total	2,421,888	2,472,088
Consumer [Member] | Allowance for loan losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	11	15
Collectively evaluated for impairment	5,070	5,527
Loans acquired with discounts related to credit quality	3
Total	5,084	5,542
Consumer [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,186	930
Collectively evaluated for impairment	1,434,156	1,403,896
Loans acquired with discounts related to credit quality	41,605	49,197
Total	1,476,947	1,454,023
Unallocated [Member] | Allowance for loan losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment
Collectively evaluated for impairment	6,928	6,796
Loans acquired with discounts related to credit quality
Total	6,928	6,796
Unallocated [Member] | Loans [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