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	Sep. 30, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Related Allowance	$ 14,840	$ 23,525
Impaired Financing Receivable, Recorded Investment	158,245	191,348
Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	14,840	23,525
Collectively evaluated for impairment	86,920	83,022
Loans acquired with discounts related to credit quality	678	7,070
Total	102,438	113,617
Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	158,245	191,348
Collectively evaluated for impairment	11,377,444	10,569,996
Loans acquired with discounts related to credit quality	629,688	806,268
Total	12,165,377	11,567,612
Commercial and Industrial [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment		11,032
Collectively evaluated for impairment	40,677	40,007
Loans acquired with discounts related to credit quality	649	512
Total	46,371	51,551
Commercial and Industrial [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment		47,303
Collectively evaluated for impairment	1,911,687	1,772,833
Loans acquired with discounts related to credit quality	144,715	201,197
Total	2,086,185	2,021,333
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Related Allowance	6,133	8,676
Impaired Financing Receivable, Recorded Investment	100,096	114,638
Commercial Real Estate [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	6,133	8,676
Collectively evaluated for impairment	30,933	27,235
Loans acquired with discounts related to credit quality	0	6,432
Total	37,066	42,343
Commercial Real Estate [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	100,096	114,638
Collectively evaluated for impairment	5,303,035	4,814,159
Loans acquired with discounts related to credit quality	429,787	543,603
Total	5,832,918	5,472,400
Residential Mortgage [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	3,216	3,735
Collectively evaluated for impairment	2,931	3,928
Loans acquired with discounts related to credit quality	29	123
Total	6,176	7,786
Residential Mortgage [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment		28,313
Collectively evaluated for impairment	2,397,704	2,456,926
Loans acquired with discounts related to credit quality	20,323	22,349
Total	2,443,367	2,507,588
Consumer [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Related Allowance	446
Individually evaluated for impairment		82
Collectively evaluated for impairment	5,334	4,274
Loans acquired with discounts related to credit quality	0	3
Total	5,780	4,359
Consumer [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment		1,094
Collectively evaluated for impairment	1,765,018	1,526,078
Loans acquired with discounts related to credit quality	34,863	39,119
Total	1,802,907	1,566,291
Unallocated [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	7,045	7,578
Loans acquired with discounts related to credit quality	0	0
Total	7,045	7,578
Unallocated [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	0	0
Loans acquired with discounts related to credit quality	0	0
Total	0	0
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Related Allowance		11,032
Impaired Financing Receivable, Recorded Investment		47,303
Commercial and Industrial [Member] | Commercial and Industrial [Member] | Allowance for Loan Losses [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Related Allowance	5,045
Commercial and Industrial [Member] | Commercial and Industrial [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	29,783
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Related Allowance	446	82
Impaired Financing Receivable, Recorded Investment		1,094
Home Equity [Member] | Consumer [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	3,026
Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Related Allowance	3,216	3,735
Impaired Financing Receivable, Recorded Investment		28,313
Residential Mortgage [Member] | Residential Mortgage [Member] | Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Impaired Financing Receivable, Recorded Investment	$ 25,340