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	Sep. 30, 2015	Dec. 31, 2014
Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 11,077	$ 12,525
Collectively evaluated for impairment	93,274	89,628
Total	104,551	102,353
Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	143,339	145,008
Collectively evaluated for impairment	13,395,687	11,607,104
Total	15,016,814	13,473,913
Residential mortgage | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,632	1,629
Collectively evaluated for impairment	2,917	3,434
Total	4,549	5,093
Residential mortgage | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	23,424	22,340
Collectively evaluated for impairment	2,839,978	2,396,704
Total	2,999,262	2,576,372
Unallocated | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	5,720	5,565
Total	5,720	5,565
Unallocated | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	0	0
Total	0	0
Commercial and industrial | Commercial and industrial | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	4,446	4,929
Collectively evaluated for impairment	42,921	38,577
Total	47,537	43,676
Commercial and industrial | Commercial and industrial | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	27,384	28,224
Collectively evaluated for impairment	2,125,254	1,931,703
Total	2,400,618	2,251,111
Commercial real estate | Commercial real estate loans | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	4,536	5,502
Collectively evaluated for impairment	37,742	37,338
Total	42,278	42,840
Commercial real estate | Commercial real estate loans | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	89,434	91,298
Collectively evaluated for impairment	6,441,869	5,438,538
Total	7,530,330	6,694,015
Consumer | Consumer | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	463	465
Collectively evaluated for impairment	3,974	4,714
Total	4,467	5,179
Consumer | Consumer | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	3,097	3,146
Collectively evaluated for impairment	1,988,586	1,840,159
Total	2,086,604	1,952,415
Loans acquired with discounts related to credit quality | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	200	200
Loans acquired with discounts related to credit quality | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	1,477,788	1,721,801
Loans acquired with discounts related to credit quality | Residential mortgage | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	0	30
Loans acquired with discounts related to credit quality | Residential mortgage | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	135,860	157,328
Loans acquired with discounts related to credit quality | Unallocated | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	0	0
Loans acquired with discounts related to credit quality | Unallocated | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	0	0
Loans acquired with discounts related to credit quality | Commercial and industrial | Commercial and industrial | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	170	170
Loans acquired with discounts related to credit quality | Commercial and industrial | Commercial and industrial | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	247,980	291,184
Loans acquired with discounts related to credit quality | Commercial real estate | Commercial real estate loans | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	0	0
Loans acquired with discounts related to credit quality | Commercial real estate | Commercial real estate loans | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	999,027	1,164,179
Loans acquired with discounts related to credit quality | Consumer | Consumer | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	30	0
Loans acquired with discounts related to credit quality | Consumer | Consumer | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with discounts related to credit quality	$ 94,921	$ 109,110