Loans and the Allowance for Credit Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment		$ 388
Allowance for loans collectively evaluated for impairment	$ 47,638	44,896
Allowance for loan commitments	607	574	$ 597	$ 571
Total allowance for credit losses	48,245	45,858	45,506	44,245
Individually evaluated for impairment	1,730	3,344
Collectively evaluated for impairment	6,780,596	6,330,023
Acquired with deteriorated credit quality	10,573	8,074
Total loans	6,792,899	6,341,441
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	4,710	3,117
Allowance for loan commitments	163	119	165	151
Total allowance for credit losses	4,873	3,236	5,622	4,499
Collectively evaluated for impairment	479,526	391,140
Acquired with deteriorated credit quality	2,164	1,457
Total loans	481,690	392,597
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment		388
Allowance for loans collectively evaluated for impairment	19,476	20,778
Allowance for loan commitments	18	26	18	17
Total allowance for credit losses	19,494	21,192	18,006	18,645
Individually evaluated for impairment	1,730	3,344
Collectively evaluated for impairment	2,699,410	2,593,393
Acquired with deteriorated credit quality	6,505	5,114
Total loans	2,707,645	2,601,851
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	11,534	9,414
Allowance for loan commitments	175	173	179	188
Total allowance for credit losses	11,709	9,587	9,413	8,600
Collectively evaluated for impairment	1,293,708	1,124,544
Acquired with deteriorated credit quality	780	783
Total loans	1,294,488	1,125,327
Deposit Overdraft [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	916	821
Total allowance for credit losses	916	821	774	760
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	3,893	3,206
Allowance for loan commitments	9	7	10	9
Total allowance for credit losses	3,902	3,213	3,727	4,115
Collectively evaluated for impairment	1,449,729	1,352,587
Acquired with deteriorated credit quality	1,100	714
Total loans	1,450,829	1,353,301
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	2,951	3,063
Allowance for loan commitments	37	37	46	44
Total allowance for credit losses	2,988	3,100	4,039	4,042
Collectively evaluated for impairment	322,594	339,163
Acquired with deteriorated credit quality		6
Total loans	322,594	339,169
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	4,158	4,497
Allowance for loan commitments	205	212	179	162
Total allowance for credit losses	4,363	4,709	$ 3,925	$ 3,584
Collectively evaluated for impairment	535,629	529,196
Acquired with deteriorated credit quality	24
Total loans	$ 535,653	$ 529,196

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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