Loans and the Allowance for Credit Losses - Summary of Age Analysis of Loan Categories (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Current	$ 10,721,084	$ 10,195,872
Total Past Due	68,149	72,113
Total loans	10,789,233	10,267,985
90 Days or More Past Due and Accruing	8,846	11,613
Loans held for sale, current	168,378	43,013
Loans held for sale	168,378	43,013
Total loans, current	10,889,462	10,238,885
Total loans	10,957,611	10,310,998
Total Non-performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	13,100
Total Past Due	27,707
Total loans	40,807
Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Current		26,174
Total Past Due		24,170
Total loans		50,344
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	639,319	641,026
Total Past Due	7,068	8,652
Total loans	646,387	649,678
90 Days or More Past Due and Accruing	706	985
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	5,681,802	5,697,874
Total Past Due	23,590	27,134
Total loans	5,705,392	5,725,008
90 Days or More Past Due and Accruing	3,001	4,735
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,698,636	1,850,806
Total Past Due	22,325	22,841
Total loans	1,720,961	1,873,647
90 Days or More Past Due and Accruing	2,863	3,643
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	305,483	370,934
Total Past Due	3,572	4,019
Total loans	309,055	374,953
90 Days or More Past Due and Accruing	377	457
Commercial Real Estate - Land and Construction [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	664,990	776,153
Total Past Due	3,287	998
Total loans	668,277	777,151
90 Days or More Past Due and Accruing	288	26
Commercial Real Estate - Improved Property [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	5,016,812	4,921,721
Total Past Due	20,303	26,136
Total loans	5,037,115	4,947,857
90 Days or More Past Due and Accruing	2,713	4,709
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	2,395,844	1,635,232
Total Past Due	11,594	9,467
Total loans	2,407,438	1,644,699
90 Days or More Past Due and Accruing	1,899	1,793
Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	9,560	21,061
Total Past Due	27,320	23,852
Total loans	36,880	44,913
TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	3,540	5,113
Total Past Due	387	318
Total loans	3,927	5,431
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	16,856	23,536
30-59 Days Past Due [Member] | Total Non-performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	693
30-59 Days Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due		1,048
30-59 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,414	3,323
Total loans	2,414
30-59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	5,458	10,736
30-59 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,614	4,421
Total loans	2,614
30-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,998	2,537
Total loans	1,998
30-59 Days Past Due [Member] | Commercial Real Estate - Land and Construction [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	582	529
30-59 Days Past Due [Member] | Commercial Real Estate - Improved Property [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	4,876	10,207
30-59 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	4,372	2,519
30-59 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	630	897
30-59 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	63	151
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	16,051	15,531
60-89 Days Past Due [Member] | Total Non-performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	618
60-89 Days Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due		1,689
60-89 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	775	621
Total loans	775
60-89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	6,394	5,760
60-89 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	5,654	5,372
Total loans	5,654
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,031	965
Total loans	1,031
60-89 Days Past Due [Member] | Commercial Real Estate - Land and Construction [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,276	121
60-89 Days Past Due [Member] | Commercial Real Estate - Improved Property [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	4,118	5,639
60-89 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,197	2,813
60-89 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	466	1,559
60-89 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	152	130
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	35,242	33,046
90 Days or More Past Due [Member] | Total Non-performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	26,396
90 Days or More Past Due [Member] | Total Impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due		21,433
90 Days or More Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,879	4,708
Total loans	3,879
90 Days or More Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	11,738	10,638
90 Days or More Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	14,057	13,048
Total loans	14,057
90 Days or More Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	543	517
Total loans	543
90 Days or More Past Due [Member] | Commercial Real Estate - Land and Construction [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	429	348
90 Days or More Past Due [Member] | Commercial Real Estate - Improved Property [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	11,309	10,290
90 Days or More Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	5,025	4,135
90 Days or More Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	26,224	21,396
90 Days or More Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 172	$ 37