Loans and the Allowance for Credit Losses - Summary of Age Analysis of Loan Categories (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Recorded Investment [Line Items]
Current	$ 10,644,956	$ 10,721,084
Total Past Due	58,356	68,149
Total loans	10,703,312	10,789,233
90 Days or More Past Due and Accruing	12,824	8,846
Loans held for sale, current	153,520	168,378
Loans held for sale	153,520	168,378
Total loans, current	10,798,476	10,889,462
Total loans	10,856,832	10,957,611
Total Non-performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	13,208	13,100
Total Past Due	24,930	27,707
Total loans	38,138	40,807
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	628,773	639,319
Total Past Due	5,245	7,068
Total loans	634,018	646,387
90 Days or More Past Due and Accruing	784	706
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	5,694,797	5,681,802
Total Past Due	17,945	23,590
Total loans	5,712,742	5,705,392
90 Days or More Past Due and Accruing	2,596	3,001
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	1,625,328	1,698,636
Total Past Due	19,094	22,325
Total loans	1,644,422	1,720,961
90 Days or More Past Due and Accruing	4,007	2,863
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	287,395	305,483
Total Past Due	2,000	3,572
Total loans	289,395	309,055
90 Days or More Past Due and Accruing	123	377
Commercial Real Estate - Land and Construction [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	639,378	664,990
Total Past Due	2,639	3,287
Total loans	642,017	668,277
90 Days or More Past Due and Accruing	641	288
Commercial Real Estate - Improved Property [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	5,055,419	5,016,812
Total Past Due	15,306	20,303
Total loans	5,070,725	5,037,115
90 Days or More Past Due and Accruing	1,955	2,713
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	2,408,663	2,395,844
Total Past Due	14,072	11,594
Total loans	2,422,735	2,407,438
90 Days or More Past Due and Accruing	5,314	1,899
Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	9,960	9,560
Total Past Due	24,615	27,320
Total loans	34,575	36,880
TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Current	3,248	3,540
Total Past Due	315	387
Total loans	3,563	3,927
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	15,338	16,856
30-59 Days Past Due [Member] | Total Non-performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,003	693
30-59 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,213	2,414
Total loans	1,213	2,414
30-59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	5,161	5,458
30-59 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,713	2,614
Total loans	2,713	2,614
30-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,428	1,998
Total loans	1,428	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	1,120	582
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	4,041	4,876
30-59 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	4,823	4,372
30-59 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	940	630
30-59 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	63	63
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	7,526	16,051
60-89 Days Past Due [Member] | Total Non-performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,259	618
60-89 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	197	775
Total loans	197	775
60-89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,743	6,394
60-89 Days Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,589	5,654
Total loans	1,589	5,654
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	340	1,031
Total loans	340	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	759	2,276
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	1,984	4,118
60-89 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	2,657	2,197
60-89 Days Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,100	466
60-89 Days Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	159	152
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	35,492	35,242
90 Days or More Past Due [Member] | Total Non-performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	22,668	26,396
90 Days or More Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	3,835	3,879
Total loans	3,835	3,879
90 Days or More Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	10,041	11,738
90 Days or More Past Due [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	14,792	14,057
Total loans	14,792	14,057
90 Days or More Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	232	543
Total loans	232	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	760	429
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	9,281	11,309
90 Days or More Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	6,592	5,025
90 Days or More Past Due [Member] | Non-Accrual Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	22,575	26,224
90 Days or More Past Due [Member] | TDRs Accruing Interest [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 93	$ 172