Loans and the Allowance for Credit Losses - Summary of Age Analysis of Loan Categories (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Current	$ 7,695,972	$ 7,610,687
Total Past Due	41,882	45,594
Total loans	7,737,854	7,656,281
90 Days or More Past Due and Accruing	2,634	4,077
Loans held for sale, current	18,649	8,994
Loans held for sale	18,649	8,994
Total loans, current	7,714,621	7,619,681
Total loans	7,756,503	7,665,275
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	584,058	591,623
Total Past Due	6,245	7,708
Total loans	590,303	599,331
90 Days or More Past Due and Accruing	533	705
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	3,864,327	3,841,425
Total Past Due	13,306	12,270
Total loans	3,877,633	3,853,695
90 Days or More Past Due and Accruing	587	175
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,617,430	1,593,519
Total Past Due	16,183	18,088
Total loans	1,633,613	1,611,607
90 Days or More Past Due and Accruing	1,173	2,820
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	333,279	322,584
Total Past Due	2,449	3,604
Total loans	335,728	326,188
90 Days or More Past Due and Accruing	244	364
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	482,016	526,660
Total Past Due	1,030	1,412
Total loans	483,046	528,072
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	3,382,311	3,314,765
Total Past Due	12,276	10,858
Total loans	3,394,587	3,325,623
90 Days or More Past Due and Accruing	587	175
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,296,878	1,261,536
Total Past Due	3,699	3,924
Total loans	1,300,577	1,265,460
90 Days or More Past Due and Accruing	97	13
Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	9,310	8,910
Total Past Due	23,588	21,790
Total loans	32,898	30,700
TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	5,273	5,586
Total Past Due	214	158
Total loans	5,487	5,744
Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	14,583	14,496
Total Past Due	23,802	21,948
Total loans	38,385	36,444
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	13,901	9,952
30-59 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,073	2,500
30-59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,262	2,328
30-59 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	5,962	2,717
30-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,682	2,084
30-59 Days Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	965	62
30-59 Days Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,297	2,266
30-59 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	922	323
30-59 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,170	337
30-59 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	10	59
30-59 Days Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,180	396
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	5,040	11,475
60-89 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	412	1,273
60-89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,531	3,600
60-89 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,449	5,001
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	506	1,007
60-89 Days Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	45	1,350
60-89 Days Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,486	2,250
60-89 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	142	594
60-89 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,169	1,370
60-89 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	146	92
60-89 Days Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,315	1,462
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	22,941	24,167
90 Days or More Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,760	3,935
90 Days or More Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	8,513	6,342
90 Days or More Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	7,772	10,370
90 Days or More Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	261	513
90 Days or More Past Due [Member] | Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	20
90 Days or More Past Due [Member] | Commercial Real Estate - Improved Property [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	8,493	6,342
90 Days or More Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,635	3,007
90 Days or More Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	20,249	20,083
90 Days or More Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	58	7
90 Days or More Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 20,307	$ 20,090