CREDIT QUALITY ASSESSMENT - Allowance for Credit or Loan Losses by Respective Loan Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 165,367	$ 56,132	$ 56,132
Initial allowance on acquired Revere PCD loans			0	18,628	18,628
Provision/ (credit) for credit losses	$ (4,204)	$ 58,686	(38,912)	83,155	85,669
Charge-offs			(3,485)	(1,039)	(1,819)
Recoveries			991	860	1,012
Net recoveries (charge-offs)			(2,494)	(179)	(807)
Balance at period end	123,961	$ 163,481	123,961	163,481	165,367
Total loans	$ 10,092,515		$ 10,092,515		$ 10,400,509
Allowance for credit losses to total loans ratio	1.23%		1.23%		1.59%
Balance of loans individually evaluated for credit loss	$ 76,917		$ 76,917		$ 97,718
Allowance related to loans evaluated individually	$ 16,283		$ 16,283		$ 11,405
Individual allowance to loans evaluated individually ratio	21.17%		21.17%		11.67%
Balance of loans collectively evaluated for credit loss	$ 10,015,598		$ 10,015,598		$ 10,302,791
Allowance related to loans evaluated collectively	$ 107,678		$ 107,678		$ 153,962
Collective allowance to loans evaluated collectively ratio	1.08%		1.08%		1.49%
Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				2,762	$ 2,762
Balance at period end	$ 0		$ 0
Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				2,983	2,983
Balance at period end	0		0
Commercial investor real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	3,712,374		3,712,374		3,634,720
Commercial owner-occupied real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	1,687,843		1,687,843		1,642,216
Commercial AD&C
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	1,126,960		1,126,960		1,050,973
Residential mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	960,527		960,527		1,105,179
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	172,869		172,869		182,619
Commercial real estate: | Commercial investor real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			57,404	18,407	18,407
Initial allowance on acquired Revere PCD loans					7,973
Provision/ (credit) for credit losses			(10,346)		33,431
Charge-offs			0		(411)
Recoveries			171		15
Net recoveries (charge-offs)			171		(396)
Balance at period end	47,229		47,229		57,404
Total loans	$ 3,712,374		$ 3,712,374		$ 3,634,720
Allowance for credit losses to total loans ratio	1.27%		1.27%		1.58%
Balance of loans individually evaluated for credit loss	$ 42,072		$ 42,072		$ 45,227
Allowance related to loans evaluated individually	$ 6,893		$ 6,893		$ 1,273
Individual allowance to loans evaluated individually ratio	16.38%		16.38%		2.81%
Balance of loans collectively evaluated for credit loss	$ 3,670,302		$ 3,670,302		$ 3,589,493
Allowance related to loans evaluated collectively	$ 40,336		$ 40,336		$ 56,131
Collective allowance to loans evaluated collectively ratio	1.10%		1.10%		1.56%
Commercial real estate: | Commercial investor real estate | Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				1,114	$ 1,114
Commercial real estate: | Commercial investor real estate | Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				(3,125)	(3,125)
Commercial real estate: | Commercial owner-occupied real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 20,061	6,884	6,884
Initial allowance on acquired Revere PCD loans					2,782
Provision/ (credit) for credit losses			(6,471)		10,008
Charge-offs			0		0
Recoveries			0		0
Net recoveries (charge-offs)			0		0
Balance at period end	$ 13,590		13,590		20,061
Total loans	$ 1,687,843		$ 1,687,843		$ 1,642,216
Allowance for credit losses to total loans ratio	0.81%		0.81%		1.22%
Balance of loans individually evaluated for credit loss	$ 8,183		$ 8,183		$ 11,561
Allowance related to loans evaluated individually	$ 143		$ 143		$ 0
Individual allowance to loans evaluated individually ratio	1.75%		1.75%		0.00%
Balance of loans collectively evaluated for credit loss	$ 1,679,660		$ 1,679,660		$ 1,630,655
Allowance related to loans evaluated collectively	$ 13,447		$ 13,447		$ 20,061
Collective allowance to loans evaluated collectively ratio	0.80%		0.80%		1.23%
Commercial real estate: | Commercial owner-occupied real estate | Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				0	$ 0
Commercial real estate: | Commercial owner-occupied real estate | Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				387	387
Commercial real estate: | Commercial AD&C
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 22,157	7,590	7,590
Initial allowance on acquired Revere PCD loans					1,248
Provision/ (credit) for credit losses			4,665		10,743
Charge-offs			0		0
Recoveries			0		0
Net recoveries (charge-offs)			0		0
Balance at period end	$ 26,822		26,822		22,157
Total loans	$ 1,126,960		$ 1,126,960		$ 1,050,973
Allowance for credit losses to total loans ratio	2.38%		2.38%		2.11%
Balance of loans individually evaluated for credit loss	$ 14,489		$ 14,489		$ 15,044
Allowance related to loans evaluated individually	$ 2,352		$ 2,352		$ 603
Individual allowance to loans evaluated individually ratio	16.23%		16.23%		4.01%
Balance of loans collectively evaluated for credit loss	$ 1,112,471		$ 1,112,471		$ 1,035,929
Allowance related to loans evaluated collectively	$ 24,470		$ 24,470		$ 21,554
Collective allowance to loans evaluated collectively ratio	2.20%		2.20%		2.08%
Commercial real estate: | Commercial AD&C | Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				0	$ 0
Commercial real estate: | Commercial AD&C | Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				2,576	2,576
Commercial Business
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	$ 8,501,543		$ 8,501,543		8,595,457
Balance of loans individually evaluated for credit loss	10,089		10,089		23,648
Allowance related to loans evaluated individually	6,895		6,895		9,529
Commercial Business | Commercial business
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			46,806	11,395	11,395
Initial allowance on acquired Revere PCD loans					6,289
Provision/ (credit) for credit losses			(18,972)		24,374
Charge-offs			(3,291)		(491)
Recoveries			298		702
Net recoveries (charge-offs)			(2,993)		211
Balance at period end	24,841		24,841		46,806
Total loans	$ 1,974,366		$ 1,974,366		$ 2,267,548
Allowance for credit losses to total loans ratio	1.26%		1.26%		2.06%
Balance of loans individually evaluated for credit loss	$ 10,089		$ 10,089		$ 23,648
Allowance related to loans evaluated individually	$ 6,895		$ 6,895		$ 9,529
Individual allowance to loans evaluated individually ratio	68.34%		68.34%		40.30%
Balance of loans collectively evaluated for credit loss	$ 1,964,277		$ 1,964,277		$ 2,243,900
Allowance related to loans evaluated collectively	$ 17,946		$ 17,946		$ 37,277
Collective allowance to loans evaluated collectively ratio	0.91%		0.91%		1.66%
Commercial Business | Commercial business | Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				1,549	$ 1,549
Commercial Business | Commercial business | Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				2,988	2,988
Residential real estate: | Residential mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 11,295	8,803	8,803
Initial allowance on acquired Revere PCD loans					243
Provision/ (credit) for credit losses			(4,214)		3,016
Charge-offs			0		(484)
Recoveries			281		105
Net recoveries (charge-offs)			281		(379)
Balance at period end	$ 7,362		7,362		11,295
Total loans	$ 960,527		$ 960,527		$ 1,105,179
Allowance for credit losses to total loans ratio	0.77%		0.77%		1.02%
Balance of loans individually evaluated for credit loss	$ 1,724		$ 1,724		$ 1,874
Allowance related to loans evaluated individually	$ 0		$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%		0.00%
Balance of loans collectively evaluated for credit loss	$ 958,803		$ 958,803		$ 1,103,305
Allowance related to loans evaluated collectively	$ 7,362		$ 7,362		$ 11,295
Collective allowance to loans evaluated collectively ratio	0.77%		0.77%		1.02%
Residential real estate: | Residential mortgage | Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				0	$ 0
Residential real estate: | Residential mortgage | Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				(388)	(388)
Residential real estate: | Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 1,502	967	967
Initial allowance on acquired Revere PCD loans					6
Provision/ (credit) for credit losses			(832)		798
Charge-offs			0		0
Recoveries			1		6
Net recoveries (charge-offs)			1		6
Balance at period end	$ 671		671		1,502
Total loans	$ 172,869		$ 172,869		$ 182,619
Allowance for credit losses to total loans ratio	0.39%		0.39%		0.82%
Balance of loans individually evaluated for credit loss	$ 0		$ 0		$ 0
Allowance related to loans evaluated individually	$ 0		$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%		0.00%
Balance of loans collectively evaluated for credit loss	$ 172,869		$ 172,869		$ 182,619
Allowance related to loans evaluated collectively	$ 671		$ 671		$ 1,502
Collective allowance to loans evaluated collectively ratio	0.39%		0.39%		0.82%
Residential real estate: | Residential construction | Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				0	$ 0
Residential real estate: | Residential construction | Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				(275)	(275)
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 6,142	2,086	2,086
Initial allowance on acquired Revere PCD loans					87
Provision/ (credit) for credit losses			(2,742)		3,299
Charge-offs			(194)		(433)
Recoveries			240		184
Net recoveries (charge-offs)			46		(249)
Balance at period end	$ 3,446		3,446		6,142
Total loans	$ 457,576		$ 457,576		$ 517,254
Allowance for credit losses to total loans ratio	0.75%		0.75%		1.19%
Balance of loans individually evaluated for credit loss	$ 360		$ 360		$ 364
Allowance related to loans evaluated individually	$ 0		$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%		0.00%
Balance of loans collectively evaluated for credit loss	$ 457,216		$ 457,216		$ 516,890
Allowance related to loans evaluated collectively	$ 3,446		$ 3,446		$ 6,142
Collective allowance to loans evaluated collectively ratio	0.75%		0.75%		1.19%
Consumer | Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				99	$ 99
Consumer | Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 820	$ 820