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
	Apr. 01, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 56,132	$ 53,486	$ 53,486
Initial allowance on acquired Revere PCD loans	$ 18,600			18,628	0
Provision for credit losses		$ 7,003	$ 1,524	90,158	3,029	4,684
Charge-offs				(1,255)	(2,101)	(2,668)
Recoveries				906	578	630
Net recoveries (charge-offs)				(349)	(1,523)	(2,038)
Balance at period end		170,314	$ 54,992	170,314	54,992	56,132
Total loans		$ 10,333,935		$ 10,333,935		$ 6,705,232
Allowance for credit losses to total loans ratio		1.65%		1.65%		0.84%
Balance of loans individually evaluated for credit loss		$ 56,122		$ 56,122		$ 24,773
Allowance related to loans evaluated individually		$ 8,263		$ 8,263		5,501
Individual allowance to loans evaluated individually ratio		14.72%		14.72%
Balance of loans collectively evaluated for credit loss		$ 10,277,813		$ 10,277,813
Allowance related to loans evaluated collectively		$ 162,051		$ 162,051		50,631
Balance of loans specifically evaluated for impairment						24,773
Allowance for loans specifically evaluated for impairment						$ 5,501
Specific allowance to specific loans ratio						22.21%
Balance of loans collectively evaluated						$ 6,667,375
Allowance for loans collectively evaluated						$ 50,631
Collective allowance to loans evaluated collectively ratio		1.58%		1.58%		0.76%
Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance of loans acquired with deteriorated credit quality						$ 13,084
Allowance for loans acquired with deteriorated credit quality						$ 0
Allowance to loan acquired with deteriorated credit quality ratio						0.00%
Initial allowance on PCD loans at adoption of ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 2,762	0	$ 0
Balance at period end		$ 2,762		2,762		2,762
Transition impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				2,983	0	0
Balance at period end						2,983
Commercial real estate: | Commercial investor real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				18,407	17,603	17,603
Initial allowance on acquired Revere PCD loans				7,973
Provision for credit losses				30,884		788
Charge-offs				(23)		0
Recoveries				6		16
Net recoveries (charge-offs)				(17)		16
Balance at period end		55,236		55,236		18,407
Total loans		$ 3,588,702		$ 3,588,702		$ 2,169,156
Allowance for credit losses to total loans ratio		1.54%		1.54%		0.85%
Balance of loans individually evaluated for credit loss		$ 27,508		$ 27,508
Allowance related to loans evaluated individually		$ 2,537		$ 2,537
Individual allowance to loans evaluated individually ratio		9.22%		9.22%
Balance of loans collectively evaluated for credit loss		$ 3,561,194		$ 3,561,194
Allowance related to loans evaluated collectively		$ 52,699		$ 52,699
Balance of loans specifically evaluated for impairment						$ 9,212
Allowance for loans specifically evaluated for impairment						$ 1,529
Specific allowance to specific loans ratio						16.60%
Balance of loans collectively evaluated						$ 2,150,400
Allowance for loans collectively evaluated						$ 16,878
Collective allowance to loans evaluated collectively ratio		1.48%		1.48%		0.78%
Commercial real estate: | Commercial investor real estate | Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance of loans acquired with deteriorated credit quality						$ 9,544
Allowance for loans acquired with deteriorated credit quality						$ 0
Allowance to loan acquired with deteriorated credit quality ratio						0.00%
Commercial real estate: | Commercial investor real estate | Initial allowance on PCD loans at adoption of ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 1,114
Balance at period end						$ 1,114
Commercial real estate: | Commercial investor real estate | Transition impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				(3,125)
Balance at period end						(3,125)
Commercial real estate: | Commercial owner-occupied real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				6,884	6,307	6,307
Initial allowance on acquired Revere PCD loans				2,782
Provision for credit losses				9,600		577
Charge-offs				0		0
Recoveries				0		0
Net recoveries (charge-offs)				0		0
Balance at period end		$ 19,653		19,653		6,884
Total loans		$ 1,652,208		$ 1,652,208		$ 1,288,677
Allowance for credit losses to total loans ratio		1.19%		1.19%		0.53%
Balance of loans individually evaluated for credit loss		$ 6,511		$ 6,511
Allowance related to loans evaluated individually		$ 22		$ 22
Individual allowance to loans evaluated individually ratio		0.34%		0.34%
Balance of loans collectively evaluated for credit loss		$ 1,645,697		$ 1,645,697
Allowance related to loans evaluated collectively		$ 19,631		$ 19,631
Balance of loans specifically evaluated for impairment						$ 4,148
Allowance for loans specifically evaluated for impairment						$ 23
Specific allowance to specific loans ratio						0.55%
Balance of loans collectively evaluated						$ 1,284,529
Allowance for loans collectively evaluated						$ 6,861
Collective allowance to loans evaluated collectively ratio		1.19%		1.19%		0.53%
Commercial real estate: | Commercial owner-occupied real estate | Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance of loans acquired with deteriorated credit quality						$ 0
Allowance for loans acquired with deteriorated credit quality						$ 0
Allowance to loan acquired with deteriorated credit quality ratio						0.00%
Commercial real estate: | Commercial owner-occupied real estate | Initial allowance on PCD loans at adoption of ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 0
Balance at period end						$ 0
Commercial real estate: | Commercial owner-occupied real estate | Transition impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				387
Balance at period end						387
Commercial real estate: | Commercial AD&C
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				7,590	5,944	5,944
Initial allowance on acquired Revere PCD loans				1,248
Provision for credit losses				7,377		1,418
Charge-offs				0		0
Recoveries				0		228
Net recoveries (charge-offs)				0		228
Balance at period end		$ 18,791		18,791		7,590
Total loans		$ 994,800		$ 994,800		$ 684,010
Allowance for credit losses to total loans ratio		1.89%		1.89%		1.11%
Balance of loans individually evaluated for credit loss		$ 1,678		$ 1,678
Allowance related to loans evaluated individually		$ 603		$ 603
Individual allowance to loans evaluated individually ratio		35.94%		35.94%
Balance of loans collectively evaluated for credit loss		$ 993,122		$ 993,122
Allowance related to loans evaluated collectively		$ 18,188		$ 18,188
Balance of loans specifically evaluated for impairment						$ 829
Allowance for loans specifically evaluated for impairment						$ 132
Specific allowance to specific loans ratio						15.92%
Balance of loans collectively evaluated						$ 683,181
Allowance for loans collectively evaluated						$ 7,458
Collective allowance to loans evaluated collectively ratio		1.83%		1.83%		1.09%
Commercial real estate: | Commercial AD&C | Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance of loans acquired with deteriorated credit quality						$ 0
Allowance for loans acquired with deteriorated credit quality						$ 0
Allowance to loan acquired with deteriorated credit quality ratio						0.00%
Commercial real estate: | Commercial AD&C | Initial allowance on PCD loans at adoption of ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 0
Balance at period end						$ 0
Commercial real estate: | Commercial AD&C | Transition impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				2,576
Balance at period end						2,576
Commercial Business
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans		$ 8,462,956		8,462,956		4,942,862
Balance of loans individually evaluated for credit loss		18,728		18,728
Allowance related to loans evaluated individually		5,101		5,101
Commercial Business | Commercial business
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				11,395	11,377	11,377
Initial allowance on acquired Revere PCD loans				6,289
Provision for credit losses				34,671		1,164
Charge-offs				(429)		(1,195)
Recoveries				696		49
Net recoveries (charge-offs)				267		(1,146)
Balance at period end		57,159		57,159		11,395
Total loans		$ 2,227,246		$ 2,227,246		$ 801,019
Allowance for credit losses to total loans ratio		2.57%		2.57%		1.42%
Balance of loans individually evaluated for credit loss		$ 18,728		$ 18,728
Allowance related to loans evaluated individually		$ 5,101		$ 5,101
Individual allowance to loans evaluated individually ratio		27.24%		27.24%
Balance of loans collectively evaluated for credit loss		$ 2,208,518		$ 2,208,518
Allowance related to loans evaluated collectively		$ 52,058		$ 52,058
Balance of loans specifically evaluated for impairment						$ 8,867
Allowance for loans specifically evaluated for impairment						$ 3,817
Specific allowance to specific loans ratio						43.05%
Balance of loans collectively evaluated						$ 789,613
Allowance for loans collectively evaluated						$ 7,578
Collective allowance to loans evaluated collectively ratio		2.36%		2.36%		0.96%
Commercial Business | Commercial business | Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance of loans acquired with deteriorated credit quality						$ 2,539
Allowance for loans acquired with deteriorated credit quality						$ 0
Allowance to loan acquired with deteriorated credit quality ratio						0.00%
Commercial Business | Commercial business | Initial allowance on PCD loans at adoption of ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 1,549
Balance at period end						$ 1,549
Commercial Business | Commercial business | Transition impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				2,988
Balance at period end						2,988
Residential real estate: | Residential mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				8,803	8,881	8,881
Initial allowance on acquired Revere PCD loans				243
Provision for credit losses				3,595		474
Charge-offs				(420)		(690)
Recoveries				78		138
Net recoveries (charge-offs)				(342)		(552)
Balance at period end		$ 11,911		11,911		8,803
Total loans		$ 1,173,857		$ 1,173,857		$ 1,149,327
Allowance for credit losses to total loans ratio		1.01%		1.01%		0.77%
Balance of loans individually evaluated for credit loss		$ 1,333		$ 1,333
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		$ 1,172,524		$ 1,172,524
Allowance related to loans evaluated collectively		$ 11,911		$ 11,911
Balance of loans specifically evaluated for impairment						$ 1,717
Allowance for loans specifically evaluated for impairment						$ 0
Specific allowance to specific loans ratio						0.00%
Balance of loans collectively evaluated						$ 1,147,602
Allowance for loans collectively evaluated						$ 8,803
Collective allowance to loans evaluated collectively ratio		1.02%		1.02%		0.77%
Residential real estate: | Residential mortgage | Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance of loans acquired with deteriorated credit quality						$ 8
Allowance for loans acquired with deteriorated credit quality						$ 0
Allowance to loan acquired with deteriorated credit quality ratio						0.00%
Residential real estate: | Residential mortgage | Initial allowance on PCD loans at adoption of ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 0
Balance at period end						$ 0
Residential real estate: | Residential mortgage | Transition impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				(388)
Balance at period end						(388)
Residential real estate: | Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				967	1,261	1,261
Initial allowance on acquired Revere PCD loans				6
Provision for credit losses				761		(302)
Charge-offs				0		0
Recoveries				5		8
Net recoveries (charge-offs)				5		8
Balance at period end		$ 1,464		1,464		967
Total loans		$ 175,123		$ 175,123		$ 146,279
Allowance for credit losses to total loans ratio		0.84%		0.84%		0.66%
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		$ 175,123		$ 175,123
Allowance related to loans evaluated collectively		$ 1,464		$ 1,464
Balance of loans specifically evaluated for impairment						$ 0
Allowance for loans specifically evaluated for impairment						$ 0
Specific allowance to specific loans ratio						0.00%
Balance of loans collectively evaluated						$ 146,279
Allowance for loans collectively evaluated						$ 967
Collective allowance to loans evaluated collectively ratio		0.84%		0.84%		0.66%
Residential real estate: | Residential construction | Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance of loans acquired with deteriorated credit quality						$ 0
Allowance for loans acquired with deteriorated credit quality						$ 0
Allowance to loan acquired with deteriorated credit quality ratio						0.00%
Residential real estate: | Residential construction | Initial allowance on PCD loans at adoption of ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 0
Balance at period end						$ 0
Residential real estate: | Residential construction | Transition impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				(275)
Balance at period end						(275)
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				2,086	$ 2,113	2,113
Initial allowance on acquired Revere PCD loans				87
Provision for credit losses				3,270		565
Charge-offs				(383)		(783)
Recoveries				121		191
Net recoveries (charge-offs)				(262)		(592)
Balance at period end		$ 6,100		6,100		2,086
Total loans		$ 521,999		$ 521,999		$ 466,764
Allowance for credit losses to total loans ratio		1.17%		1.17%		0.45%
Balance of loans individually evaluated for credit loss		$ 364		$ 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		$ 521,635		$ 521,635
Allowance related to loans evaluated collectively		$ 6,100		$ 6,100
Balance of loans specifically evaluated for impairment						$ 0
Allowance for loans specifically evaluated for impairment						$ 0
Specific allowance to specific loans ratio						0.00%
Balance of loans collectively evaluated						$ 465,771
Allowance for loans collectively evaluated						$ 2,086
Collective allowance to loans evaluated collectively ratio		1.17%		1.17%		0.45%
Consumer | Financial Asset Acquired with Credit Deterioration
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance of loans acquired with deteriorated credit quality						$ 993
Allowance for loans acquired with deteriorated credit quality						$ 0
Allowance to loan acquired with deteriorated credit quality ratio						0.00%
Consumer | Initial allowance on PCD loans at adoption of ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 99
Balance at period end						$ 99
Consumer | Transition impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period				$ 820
Balance at period end						$ 820