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	Dec. 31, 2012	Dec. 31, 2011
Allowance for Loan and Lease Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 31,013	$ 23,243
Collectively evaluated for impairment	89,695	97,031
Loans acquired with discounts related to credit quality	9,492	13,528
Total	130,200	133,802
Allowance for Loan and Lease Losses [Member] | Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	12,088	11,105
Collectively evaluated for impairment	44,877	51,588
Loans acquired with discounts related to credit quality	7,405	10,956
Total	64,370	73,649
Allowance for Loan and Lease Losses [Member] | Commercial real estate loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	16,581	8,516
Collectively evaluated for impairment	25,463	23,611
Loans acquired with discounts related to credit quality	2,025	2,510
Total	44,069	34,637
Allowance for Loan and Lease Losses [Member] | Residential mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	2,329	3,577
Collectively evaluated for impairment	7,032	5,481
Loans acquired with discounts related to credit quality	62	62
Total	9,423	9,120
Allowance for Loan and Lease Losses [Member] | Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	15	45
Collectively evaluated for impairment	5,527	8,632
Loans acquired with discounts related to credit quality
Total	5,542	8,677
Allowance for Loan and Lease Losses [Member] | Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment
Collectively evaluated for impairment	6,796	7,719
Loans acquired with discounts related to credit quality
Total	6,796	7,719
Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	206,734	184,075
Collectively evaluated for impairment	9,648,401	9,343,722
Loans acquired with discounts related to credit quality	1,167,664	271,844
Total	11,022,799	9,799,641
Loans [Member] | Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	49,697	54,858
Collectively evaluated for impairment	1,783,046	1,823,529
Loans acquired with discounts related to credit quality	298,600	83,742
Total	2,131,343	1,962,129
Loans [Member] | Commercial real estate loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	131,216	107,970
Collectively evaluated for impairment	4,040,723	3,877,122
Loans acquired with discounts related to credit quality	793,406	167,625
Total	4,965,345	4,152,717
Loans [Member] | Residential mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	24,891	21,005
Collectively evaluated for impairment	2,420,736	2,264,585
Loans acquired with discounts related to credit quality	26,461	15,546
Total	2,472,088	2,301,136
Loans [Member] | Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	930	242
Collectively evaluated for impairment	1,403,896	1,378,486
Loans acquired with discounts related to credit quality	49,197	4,931
Total	1,454,023	1,383,659
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