Loans and the Allowance for Credit Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	$ 1,398	$ 1,521
Allowance for loans collectively evaluated for impairment	41,930	40,189
Allowance for loan commitments	585	613	$ 805	$ 455
Total allowance for credit losses	43,913	42,323	44,224	45,109
Individually evaluated for impairment	7,061	8,903
Collectively evaluated for impairment	5,155,253	5,047,650
Acquired with deteriorated credit quality	7,518	9,289
Total Loans	5,169,832	5,065,842
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	5,645	4,390
Allowance for loan commitments	147	157	207	194
Total allowance for credit losses	5,792	4,547	5,310	5,848
Collectively evaluated for impairment	432,663	343,832
Acquired with deteriorated credit quality		916
Total Loans	432,663	344,748
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	504	668
Allowance for loans collectively evaluated for impairment	13,525	14,080
Allowance for loan commitments	13	26	20	10
Total allowance for credit losses	14,042	14,774	17,248	17,583
Individually evaluated for impairment	3,012	4,031
Collectively evaluated for impairment	1,840,005	1,899,738
Acquired with deteriorated credit quality	7,518	7,864
Total Loans	1,850,535	1,911,633
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans individually evaluated for impairment	894	853
Allowance for loans collectively evaluated for impairment	10,481	9,149
Allowance for loan commitments	244	260	413	112
Total allowance for credit losses	11,619	10,262	9,604	9,175
Individually evaluated for impairment	4,049	4,872
Collectively evaluated for impairment	810,006	732,957
Acquired with deteriorated credit quality		49
Total Loans	814,055	737,878
Deposit Overdraft [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	679	342
Total allowance for credit losses	679	342	543	575
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	4,330	4,582
Allowance for loan commitments	8	7	12	9
Total allowance for credit losses	4,338	4,589	4,776	5,391
Collectively evaluated for impairment	1,242,015	1,247,639
Acquired with deteriorated credit quality		161
Total Loans	1,242,015	1,247,800
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	4,362	4,763
Allowance for loan commitments	46	46	40	40
Total allowance for credit losses	4,408	4,809	3,942	4,118
Collectively evaluated for impairment	395,377	406,622
Acquired with deteriorated credit quality		272
Total Loans	395,377	406,894
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans collectively evaluated for impairment	2,908	2,883
Allowance for loan commitments	127	117	113	90
Total allowance for credit losses	3,035	3,000	$ 2,801	$ 2,419
Collectively evaluated for impairment	435,187	416,862
Acquired with deteriorated credit quality		27
Total Loans	$ 435,187	$ 416,889