CREDIT QUALITY ASSESSMENT - Allowance for Credit or Loan Losses by Respective Loan Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 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	$ (8,229)	$ 7,003	(47,141)	90,158	85,669
Charge-offs			(11,496)	(1,255)	(1,819)
Recoveries			1,190	906	1,012
Net recoveries (charge-offs)			(10,306)	(349)	(807)
Balance at period end	107,920	$ 170,314	107,920	170,314	165,367
Total loans	$ 9,721,348		$ 9,721,348		$ 10,400,509
Allowance for credit losses to total loans ratio	1.11%		1.11%		1.59%
Balance of loans individually evaluated for credit loss	$ 38,423		$ 38,423		$ 97,718
Allowance related to loans evaluated individually	$ 7,537		$ 7,537		$ 11,405
Individual allowance to loans evaluated individually ratio	19.62%		19.62%		11.67%
Balance of loans collectively evaluated for credit loss	$ 9,682,925		$ 9,682,925		$ 10,302,791
Allowance related to loans evaluated collectively	$ 100,383		$ 100,383		$ 153,962
Collective allowance to loans evaluated collectively ratio	1.04%		1.04%		1.49%
Revision of Prior Period, Reclassification, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			$ 0	2,762	$ 2,762
Balance at period end					0
Revision of Prior Period, Accounting Standards Update, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period			0	2,983	2,983
Balance at period end					0
Commercial investor real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	$ 3,743,698		3,743,698		3,634,720
Commercial owner-occupied real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	1,661,092		1,661,092		1,642,216
Commercial AD&C
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	1,177,949		1,177,949		1,050,973
Residential mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	911,997		911,997		1,105,179
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	181,319		181,319		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			(15,822)		33,431
Charge-offs			(5,802)		(411)
Recoveries			176		15
Net recoveries (charge-offs)			(5,626)		(396)
Balance at period end	35,956		35,956		57,404
Total loans	$ 3,743,698		$ 3,743,698		$ 3,634,720
Allowance for credit losses to total loans ratio	0.96%		0.96%		1.58%
Balance of loans individually evaluated for credit loss	$ 15,386		$ 15,386		$ 45,227
Allowance related to loans evaluated individually	$ 203		$ 203		$ 1,273
Individual allowance to loans evaluated individually ratio	1.32%		1.32%		2.81%
Balance of loans collectively evaluated for credit loss	$ 3,728,312		$ 3,728,312		$ 3,589,493
Allowance related to loans evaluated collectively	$ 35,753		$ 35,753		$ 56,131
Collective allowance to loans evaluated collectively ratio	0.96%		0.96%		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			(8,165)		10,008
Charge-offs			(136)		0
Recoveries			0		0
Net recoveries (charge-offs)			(136)		0
Balance at period end	$ 11,760		11,760		20,061
Total loans	$ 1,661,092		$ 1,661,092		$ 1,642,216
Allowance for credit losses to total loans ratio	0.71%		0.71%		1.22%
Balance of loans individually evaluated for credit loss	$ 9,854		$ 9,854		$ 11,561
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	$ 1,651,238		$ 1,651,238		$ 1,630,655
Allowance related to loans evaluated collectively	$ 11,760		$ 11,760		$ 20,061
Collective allowance to loans evaluated collectively ratio	0.71%		0.71%		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			8,538		10,743
Charge-offs			(2,007)		0
Recoveries			0		0
Net recoveries (charge-offs)			(2,007)		0
Balance at period end	$ 28,688		28,688		22,157
Total loans	$ 1,177,949		$ 1,177,949		$ 1,050,973
Allowance for credit losses to total loans ratio	2.44%		2.44%		2.11%
Balance of loans individually evaluated for credit loss	$ 1,022		$ 1,022		$ 15,044
Allowance related to loans evaluated individually	$ 509		$ 509		$ 603
Individual allowance to loans evaluated individually ratio	49.80%		49.80%		4.01%
Balance of loans collectively evaluated for credit loss	$ 1,176,927		$ 1,176,927		$ 1,035,929
Allowance related to loans evaluated collectively	$ 28,179		$ 28,179		$ 21,554
Collective allowance to loans evaluated collectively ratio	2.39%		2.39%		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	$ 8,177,267		$ 8,177,267		8,595,457
Balance of loans individually evaluated for credit loss	10,088		10,088		23,648
Allowance related to loans evaluated individually	6,825		6,825		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			(20,721)		24,374
Charge-offs			(3,315)		(491)
Recoveries			375		702
Net recoveries (charge-offs)			(2,940)		211
Balance at period end	23,145		23,145		46,806
Total loans	$ 1,594,528		$ 1,594,528		$ 2,267,548
Allowance for credit losses to total loans ratio	1.45%		1.45%		2.06%
Balance of loans individually evaluated for credit loss	$ 10,088		$ 10,088		$ 23,648
Allowance related to loans evaluated individually	$ 6,825		$ 6,825		$ 9,529
Individual allowance to loans evaluated individually ratio	67.65%		67.65%		40.30%
Balance of loans collectively evaluated for credit loss	$ 1,584,440		$ 1,584,440		$ 2,243,900
Allowance related to loans evaluated collectively	$ 16,320		$ 16,320		$ 37,277
Collective allowance to loans evaluated collectively ratio	1.03%		1.03%		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			(6,607)		3,016
Charge-offs			0		(484)
Recoveries			330		105
Net recoveries (charge-offs)			330		(379)
Balance at period end	$ 5,018		5,018		11,295
Total loans	$ 911,997		$ 911,997		$ 1,105,179
Allowance for credit losses to total loans ratio	0.55%		0.55%		1.02%
Balance of loans individually evaluated for credit loss	$ 1,716		$ 1,716		$ 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	$ 910,281		$ 910,281		$ 1,103,305
Allowance related to loans evaluated collectively	$ 5,018		$ 5,018		$ 11,295
Collective allowance to loans evaluated collectively ratio	0.55%		0.55%		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			(849)		798
Charge-offs			0		0
Recoveries			3		6
Net recoveries (charge-offs)			3		6
Balance at period end	$ 656		656		1,502
Total loans	$ 181,319		$ 181,319		$ 182,619
Allowance for credit losses to total loans ratio	0.36%		0.36%		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	$ 181,319		$ 181,319		$ 182,619
Allowance related to loans evaluated collectively	$ 656		$ 656		$ 1,502
Collective allowance to loans evaluated collectively ratio	0.36%		0.36%		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			(3,515)		3,299
Charge-offs			(236)		(433)
Recoveries			306		184
Net recoveries (charge-offs)			70		(249)
Balance at period end	$ 2,697		2,697		6,142
Total loans	$ 450,765		$ 450,765		$ 517,254
Allowance for credit losses to total loans ratio	0.60%		0.60%		1.19%
Balance of loans individually evaluated for credit loss	$ 357		$ 357		$ 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	$ 450,408		$ 450,408		$ 516,890
Allowance related to loans evaluated collectively	$ 2,697		$ 2,697		$ 6,142
Collective allowance to loans evaluated collectively ratio	0.60%		0.60%		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