CREDIT QUALITY ASSESSMENT - Allowance for Credit or Loan Losses by Respective Loan Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 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			18,628
Provision/ (credit) for credit losses	(34,708)	24,469	85,669
Charge-offs	(743)	(654)	(1,819)
Recoveries	445	108	1,012
Net recoveries (charge-offs)	(298)	(546)	(807)
Balance at period end	130,361	85,800	165,367
Total loans	$ 10,446,866		$ 10,400,509
Allowance for credit losses to total loans ratio	1.25%		1.59%
Balance of loans individually evaluated for credit loss	$ 81,998		$ 97,718
Allowance related to loans evaluated individually	$ 13,101		$ 11,405
Individual allowance to loans evaluated individually ratio	15.98%		11.67%
Balance of loans collectively evaluated for credit loss	$ 10,364,868		$ 10,302,791
Allowance related to loans evaluated collectively	$ 117,260		$ 153,962
Collective allowance to loans evaluated collectively ratio	1.13%		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
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
Commercial investor real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	$ 3,652,418		3,634,720
Commercial owner-occupied real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	1,644,848		1,642,216
Commercial AD&C
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	1,051,013		1,050,973
Residential mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	1,022,546		1,105,179
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	171,028		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	(8,981)		33,431
Charge-offs	0		(411)
Recoveries	27		15
Net recoveries (charge-offs)	27		(396)
Balance at period end	48,450		57,404
Total loans	$ 3,652,418		$ 3,634,720
Allowance for credit losses to total loans ratio	1.33%		1.58%
Balance of loans individually evaluated for credit loss	$ 42,776		$ 45,227
Allowance related to loans evaluated individually	$ 2,988		$ 1,273
Individual allowance to loans evaluated individually ratio	6.99%		2.81%
Balance of loans collectively evaluated for credit loss	$ 3,609,642		$ 3,589,493
Allowance related to loans evaluated collectively	$ 45,462		$ 56,131
Collective allowance to loans evaluated collectively ratio	1.26%		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	(5,594)		10,008
Charge-offs	0		0
Recoveries	0		0
Net recoveries (charge-offs)	0		0
Balance at period end	14,467		20,061
Total loans	$ 1,644,848		$ 1,642,216
Allowance for credit losses to total loans ratio	0.88%		1.22%
Balance of loans individually evaluated for credit loss	$ 8,316		$ 11,561
Allowance related to loans evaluated individually	$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Balance of loans collectively evaluated for credit loss	$ 1,636,532		$ 1,630,655
Allowance related to loans evaluated collectively	$ 14,467		$ 20,061
Collective allowance to loans evaluated collectively ratio	0.88%		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	760		10,743
Charge-offs	0		0
Recoveries	0		0
Net recoveries (charge-offs)	0		0
Balance at period end	22,917		22,157
Total loans	$ 1,051,013		$ 1,050,973
Allowance for credit losses to total loans ratio	2.18%		2.11%
Balance of loans individually evaluated for credit loss	$ 14,975		$ 15,044
Allowance related to loans evaluated individually	$ 603		$ 603
Individual allowance to loans evaluated individually ratio	4.03%		4.01%
Balance of loans collectively evaluated for credit loss	$ 1,036,038		$ 1,035,929
Allowance related to loans evaluated collectively	$ 22,314		$ 21,554
Collective allowance to loans evaluated collectively ratio	2.15%		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	$ 8,759,388		8,595,457
Balance of loans individually evaluated for credit loss	13,831		23,648
Allowance related to loans evaluated individually	9,510		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	(14,343)		24,374
Charge-offs	(650)		(491)
Recoveries	16		702
Net recoveries (charge-offs)	(634)		211
Balance at period end	31,829		46,806
Total loans	$ 2,411,109		$ 2,267,548
Allowance for credit losses to total loans ratio	1.32%		2.06%
Balance of loans individually evaluated for credit loss	$ 13,831		$ 23,648
Allowance related to loans evaluated individually	$ 9,510		$ 9,529
Individual allowance to loans evaluated individually ratio	68.76%		40.30%
Balance of loans collectively evaluated for credit loss	$ 2,397,278		$ 2,243,900
Allowance related to loans evaluated collectively	$ 22,319		$ 37,277
Collective allowance to loans evaluated collectively ratio	0.93%		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	(4,018)		3,016
Charge-offs	0		(484)
Recoveries	270		105
Net recoveries (charge-offs)	270		(379)
Balance at period end	7,547		11,295
Total loans	$ 1,022,546		$ 1,105,179
Allowance for credit losses to total loans ratio	0.74%		1.02%
Balance of loans individually evaluated for credit loss	$ 1,737		$ 1,874
Allowance related to loans evaluated individually	$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Balance of loans collectively evaluated for credit loss	$ 1,020,809		$ 1,103,305
Allowance related to loans evaluated collectively	$ 7,547		$ 11,295
Collective allowance to loans evaluated collectively ratio	0.74%		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	0		6
Net recoveries (charge-offs)	0		6
Balance at period end	653		1,502
Total loans	$ 171,028		$ 182,619
Allowance for credit losses to total loans ratio	0.38%		0.82%
Balance of loans individually evaluated for credit loss	$ 0		$ 0
Allowance related to loans evaluated individually	$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Balance of loans collectively evaluated for credit loss	$ 171,028		$ 182,619
Allowance related to loans evaluated collectively	$ 653		$ 1,502
Collective allowance to loans evaluated collectively ratio	0.38%		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	(1,683)		3,299
Charge-offs	(93)		(433)
Recoveries	132		184
Net recoveries (charge-offs)	39		(249)
Balance at period end	4,498		6,142
Total loans	$ 493,904		$ 517,254
Allowance for credit losses to total loans ratio	0.91%		1.19%
Balance of loans individually evaluated for credit loss	$ 363		$ 364
Allowance related to loans evaluated individually	$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Balance of loans collectively evaluated for credit loss	$ 493,541		$ 516,890
Allowance related to loans evaluated collectively	$ 4,498		$ 6,142
Collective allowance to loans evaluated collectively ratio	0.91%		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