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, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 33,025,000	$ 14,561,000
Collectively evaluated for impairment	118,834,000	106,295,000
Allowance for loan losses	151,859,000	120,856,000	$ 114,419,000
Individually evaluated for impairment	156,625,000	164,166,000
Collectively evaluated for impairment	20,688,758,000	16,780,199,000
Total loans	25,035,469,000	18,331,580,000
Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses	0	0
Total loans	4,190,086,000	1,387,215,000
Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	29,684,000	11,044,000
Collectively evaluated for impairment	61,272,000	46,188,000
Allowance for loan losses	90,956,000	57,232,000	50,820,000
Individually evaluated for impairment	97,852,000	85,499,000
Collectively evaluated for impairment	3,492,523,000	2,463,566,000
Total loans	4,331,032,000	2,741,425,000
Commercial and Industrial | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	740,657,000	192,360,000
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	2,628,000	2,735,000
Collectively evaluated for impairment	47,022,000	52,219,000
Allowance for loan losses	49,650,000	54,954,000	55,851,000
Individually evaluated for impairment	43,598,000	60,851,000
Collectively evaluated for impairment	10,991,059,000	9,310,964,000
Total loans	13,895,407,000	10,347,882,000
Commercial Real Estate | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	2,860,750,000	976,067,000
Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	600,000	718,000
Collectively evaluated for impairment	4,441,000	2,887,000
Allowance for loan losses	5,041,000	3,605,000	3,702,000
Individually evaluated for impairment	13,904,000	14,056,000
Collectively evaluated for impairment	3,669,080,000	2,703,688,000
Total loans	4,111,400,000	2,859,035,000
Residential Mortgage | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	428,416,000	141,291,000
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	113,000	64,000
Collectively evaluated for impairment	6,099,000	5,001,000
Allowance for loan losses	6,212,000	5,065,000	$ 4,046,000
Individually evaluated for impairment	1,271,000	3,760,000
Collectively evaluated for impairment	2,536,096,000	2,301,981,000
Total loans	2,697,630,000	2,383,238,000
Consumer | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 160,263,000	$ 77,497,000