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 ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment, allowance for loan losses	$ 10,531,000	$ 8,782,000
Collectively evaluated for impairment, allowance for loan losses	103,888,000	97,396,000
Allowance for loan losses	114,419,000	106,178,000	$ 102,353,000
Individually evaluated for impairment	114,819,000	132,388,000
Collectively evaluated for impairment	15,349,782,000	13,670,248,000
Total loans	17,236,103,000	16,043,107,000
Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses	0	0
Total loans	1,771,502,000	2,240,471,000
Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment, allowance for loan losses	5,864,000	3,439,000
Collectively evaluated for impairment, allowance for loan losses	44,956,000	45,328,000
Allowance for loan losses	50,820,000	48,767,000	43,676,000
Individually evaluated for impairment	30,640,000	25,714,000
Collectively evaluated for impairment	2,326,378,000	2,130,835,000
Total loans	2,638,195,000	2,540,491,000
Commercial and Industrial | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	281,177,000	383,942,000
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment, allowance for loan losses	3,872,000	3,671,000
Collectively evaluated for impairment, allowance for loan losses	51,979,000	44,335,000
Allowance for loan losses	55,851,000	48,006,000	42,840,000
Individually evaluated for impairment	62,289,000	81,930,000
Collectively evaluated for impairment	8,276,305,000	6,595,296,000
Total loans	9,544,613,000	8,179,583,000
Commercial Real Estate | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,206,019,000	1,502,357,000
Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment, allowance for loan losses	725,000	1,377,000
Collectively evaluated for impairment, allowance for loan losses	2,977,000	3,248,000
Allowance for loan losses	3,702,000	4,625,000	5,093,000
Individually evaluated for impairment	18,356,000	22,612,000
Collectively evaluated for impairment	2,665,839,000	2,889,467,000
Total loans	2,867,918,000	3,130,541,000
Residential Mortgage | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	183,723,000	218,462,000
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment, allowance for loan losses	70,000	295,000
Collectively evaluated for impairment, allowance for loan losses	3,976,000	4,485,000
Allowance for loan losses	4,046,000	4,780,000	$ 5,179,000
Individually evaluated for impairment	3,534,000	2,132,000
Collectively evaluated for impairment	2,081,260,000	2,054,650,000
Total loans	2,185,377,000	2,192,492,000
Consumer | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 100,583,000	$ 135,710,000