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	Sep. 30, 2012	Dec. 31, 2011
Allowance for Loan and Lease Losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 23,931	$ 23,243
Collectively evaluated for impairment	95,824	97,031
Loans acquired with discounts related to credit quality	9,492	13,528
Total	129,247	133,802
Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	200,263	184,075
Collectively evaluated for impairment	9,674,322	9,343,722
Loans acquired with discounts related to credit quality	1,274,353	271,844
Total	11,148,938	9,799,641
Commercial and Industrial | Allowance for Loan and Lease Losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	9,051	11,105
Collectively evaluated for impairment	49,063	51,588
Loans acquired with discounts related to credit quality	7,405	10,956
Total	65,519	73,649
Commercial and Industrial | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	42,988	54,858
Collectively evaluated for impairment	1,804,662	1,823,529
Loans acquired with discounts related to credit quality	322,926	83,742
Total	2,170,576	1,962,129
Commercial Real Estate | Allowance for Loan and Lease Losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	11,775	8,516
Collectively evaluated for impairment	27,470	23,611
Loans acquired with discounts related to credit quality	2,025	2,510
Total	41,270	34,637
Commercial Real Estate | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	129,094	107,970
Collectively evaluated for impairment	4,021,161	3,877,122
Loans acquired with discounts related to credit quality	872,077	167,625
Total	5,022,332	4,152,717
Residential Mortgage | Allowance for Loan and Lease Losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	3,090	3,577
Collectively evaluated for impairment	6,704	5,481
Loans acquired with discounts related to credit quality	62	62
Total	9,856	9,120
Residential Mortgage | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	26,275	21,005
Collectively evaluated for impairment	2,456,088	2,264,585
Loans acquired with discounts related to credit quality	28,951	15,546
Total	2,511,314	2,301,136
Consumer | Allowance for Loan and Lease Losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	15	45
Collectively evaluated for impairment	5,648	8,632
Loans acquired with discounts related to credit quality
Total	5,663	8,677
Consumer | Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,906	242
Collectively evaluated for impairment	1,392,411	1,378,486
Loans acquired with discounts related to credit quality	50,399	4,931
Total	1,444,716	1,383,659
Unallocated | Allowance for Loan and Lease Losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment
Collectively evaluated for impairment	6,939	7,719
Loans acquired with discounts related to credit quality
Total	6,939	7,719
Unallocated | Loans
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