Loans and the Allowance for Credit Losses - Summary of Age Analysis of Loan Categories (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Current	$ 6,753,798	$ 6,299,397
Total Past Due	39,101	42,044
Total loans	6,792,899	6,341,441
90 Days or More Past Due and Accruing	1,881	2,726
Loans held for sale, current	12,053	20,320
Loans held for sale	12,053	20,320
Total loans, current	6,765,851	6,319,717
Total loans	6,804,952	6,361,761
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	529,625	522,584
Total Past Due	6,028	6,612
Total loans	535,653	529,196
90 Days or More Past Due and Accruing	477	742
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	3,178,874	2,981,893
Total Past Due	10,461	12,555
Total loans	3,189,335	2,994,448
90 Days or More Past Due and Accruing	422	243
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,435,731	1,338,240
Total Past Due	15,098	15,061
Total loans	1,450,829	1,353,301
90 Days or More Past Due and Accruing	255	1,113
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	319,157	334,723
Total Past Due	3,437	4,446
Total loans	322,594	339,169
90 Days or More Past Due and Accruing	508	608
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	481,156	392,189
Total Past Due	534	408
Total loans	481,690	392,597
90 Days or More Past Due and Accruing	172
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	2,697,718	2,589,704
Total Past Due	9,927	12,147
Total loans	2,707,645	2,601,851
90 Days or More Past Due and Accruing	250	243
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,290,411	1,121,957
Total Past Due	4,077	3,370
Total loans	1,294,488	1,125,327
90 Days or More Past Due and Accruing	219	20
Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	8,525	9,195
Total Past Due	23,456	27,630
Total loans	31,981	36,825
TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	6,053	6,055
Total Past Due	407	516
Total loans	6,460	6,571
Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	14,578	15,250
Total Past Due	23,863	28,146
Total loans	38,441	43,396
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	11,526	10,034
30-59 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,593	2,135
30-59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,695	374
30-59 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	5,469	4,487
30-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,025	2,466
30-59 Days Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	287
30-59 Days Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,408	374
30-59 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	744	572
30-59 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,316	1,782
30-59 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	139	348
30-59 Days Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,455	2,130
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	5,274	5,469
60-89 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,232	683
60-89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	240	1,372
60-89 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,798	2,376
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	569	842
60-89 Days Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	75	172
60-89 Days Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	165	1,200
60-89 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	435	196
60-89 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,734	2,033
60-89 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	254	168
60-89 Days Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,988	2,201
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	22,301	26,541
90 Days or More Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,203	3,794
90 Days or More Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	8,526	10,809
90 Days or More Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	6,831	8,198
90 Days or More Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	843	1,138
90 Days or More Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	172	236
90 Days or More Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	8,354	10,573
90 Days or More Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,898	2,602
90 Days or More Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	20,406	23,815
90 Days or More Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	14
90 Days or More Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 20,420	$ 23,815