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	Jun. 30, 2015	Dec. 31, 2014
Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 10,651	$ 12,525
Collectively evaluated for impairment	91,984	89,628
Total	102,835	102,353
Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	146,367	145,008
Collectively evaluated for impairment	12,762,455	11,607,104
Total	14,480,294	13,473,913
Residential mortgage | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,427	1,629
Collectively evaluated for impairment	3,598	3,434
Total	5,055	5,093
Residential mortgage | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	22,889	22,340
Collectively evaluated for impairment	2,536,242	2,396,704
Total	2,704,081	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	6,339	5,565
Total	6,339	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	3,724	4,929
Collectively evaluated for impairment	37,820	38,577
Total	41,714	43,676
Commercial and industrial | Commercial and industrial | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	24,839	28,224
Collectively evaluated for impairment	2,086,193	1,931,703
Total	2,372,031	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	4,682	5,502
Collectively evaluated for impairment	39,503	37,338
Total	44,185	42,840
Commercial real estate | Commercial real estate loans | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	93,926	91,298
Collectively evaluated for impairment	6,211,168	5,438,538
Total	7,369,217	6,694,015
Consumer | Consumer | Allowance for Loan Losses:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	818	465
Collectively evaluated for impairment	4,724	4,714
Total	5,542	5,179
Consumer | Consumer | Loans:
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	4,713	3,146
Collectively evaluated for impairment	1,928,852	1,840,159
Total	2,034,965	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	1,571,472	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	30	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	144,950	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	260,999	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	1,064,123	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	0	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	$ 101,400	$ 109,110