Loans and Allowance for Loan Losses - Summary of Shows the Age Analysis of Past Due Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 58,008	$ 47,622
Current	9,858,467	9,682,903
Total loans	9,916,475	9,730,525	$ 9,087,103
Recorded Investment > 90 Days and Accruing	1,679	2,247
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	34,113	18,251
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,179	7,250
90 or More Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	20,716	22,121
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due	10,637	2,196
Current	1,975,729	1,992,820
Total loans	1,986,366	1,995,016	1,771,122
Recorded Investment > 90 Days and Accruing	252	848
Commercial and industrial | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	9,460	4
Commercial and industrial | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3	268
Commercial and industrial | 90 or More Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,174	1,924
Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	4,628	2,101
Current	3,672,313	3,571,529
Total loans	3,676,941	3,573,630	3,523,721
Recorded Investment > 90 Days and Accruing	1,138	1,111
Commercial real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,225	0
Commercial real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	556
Commercial real estate | 90 or More Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,403	1,545
Commercial construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	249,416	305,708
Total loans	249,416	305,708	293,135
Recorded Investment > 90 Days and Accruing	0	0
Commercial construction | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Commercial construction | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Commercial construction | 90 or More Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Business banking
Financing Receivable, Past Due [Line Items]
Total Past Due	23,216	18,786
Current	1,489,835	1,320,378
Total loans	1,513,051	1,339,164
Recorded Investment > 90 Days and Accruing	0	0
Business banking | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	13,299	5,279
Business banking | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,404	3,311
Business banking | 90 or More Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	8,513	10,196
Residential real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	12,866	16,605
Current	1,393,644	1,354,352
Total loans	1,406,510	1,370,957
Recorded Investment > 90 Days and Accruing	280	279
Residential real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	5,738	9,184
Residential real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	857	2,517
Residential real estate | 90 or More Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	6,271	4,904
Consumer home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	4,918	5,205
Current	827,548	863,065
Total loans	832,466	868,270
Recorded Investment > 90 Days and Accruing	9	9
Consumer home equity | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,416	1,806
Consumer home equity | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	496	364
Consumer home equity | 90 or More Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,006	3,035
Other consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	1,743	2,729
Current	249,982	275,051
Total loans	251,725	277,780	$ 369,652
Recorded Investment > 90 Days and Accruing	0	0
Other consumer | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	975	1,978
Other consumer | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	419	234
Other consumer | 90 or More Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 349	$ 517