CREDIT QUALITY ASSESSMENT (Activity in Allowance for Credit Losses or Loan Losses by Respective Loan Portfolio Segment) (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 56,132,000	$ 53,486,000	$ 53,486,000
Initial allowance on acquired Revere PCD loans			18,628,000	0
Provision for credit losses	$ 58,686,000	$ 1,633,000	83,155,000	1,505,000	4,684,000
Charge-offs			(1,039,000)	(1,197,000)	(2,668,000)
Recoveries			860,000	230,000	630,000
Net recoveries (charge-offs)			(179,000)	(967,000)	(2,038,000)
Balance at end of period	163,481,000	$ 54,024,000	163,481,000	54,024,000	56,132,000
Total loans	$ 10,343,043,000		$ 10,343,043,000		$ 6,705,232,000
Allowance for loan losses to total loans ratio	1.58%		1.58%		0.84%
Balance of loans individually evaluated for credit loss	$ 60,528,000		$ 60,528,000		$ 24,773,000
Balance of loans specifically evaluated for impairment					24,773,000
Allowance related to loans evaluated individually	$ 8,827,000		$ 8,827,000		5,501,000
Allowance for loans specifically evaluated for impairment					$ 5,501,000
Individual allowance to loans evaluated individually ratio	14.58%		14.58%
Specific allowance to specific loans ratio					22.21%
Balance of loans collectively evaluated for credit loss	$ 10,282,515,000		$ 10,282,515,000
Balance of loans collectively evaluated					$ 6,667,375,000
Allowance related to loans evaluated collectively	$ 154,654,000		$ 154,654,000		50,631,000
Allowance for loans collectively evaluated					$ 50,631,000
Collective allowance to collective loans ratio	1.50%		1.50%		0.76%
Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance of loans acquired					$ 13,084,000
Allowance for loans acquired					$ 0
Allowance to loans acquired with deteriorated credity quality ratio					0.00%
Initial allowance on purchased credit deteriorated loans at adoption of ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 2,762,000	0	$ 0
Balance at end of period					2,762,000
Transition impact of adopting ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			2,983,000	0	0
Balance at end of period					2,983,000
Commercial [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			11,395,000	11,377,000	11,377,000
Initial allowance on acquired Revere PCD loans			6,289,000
Provision for credit losses			36,060,000		1,164,000
Charge-offs			(339,000)		(1,195,000)
Recoveries			694,000		49,000
Net recoveries (charge-offs)			355,000		(1,146,000)
Balance at end of period	$ 58,636,000		58,636,000		11,395,000
Total loans	$ 2,222,810,000		$ 2,222,810,000		$ 801,019,000
Allowance for loan losses to total loans ratio	2.64%		2.64%		1.42%
Balance of loans individually evaluated for credit loss	$ 20,953,000		$ 20,953,000
Balance of loans specifically evaluated for impairment					$ 8,867,000
Allowance related to loans evaluated individually	$ 5,488,000		$ 5,488,000
Allowance for loans specifically evaluated for impairment					$ 3,817,000
Individual allowance to loans evaluated individually ratio	26.19%		26.19%
Specific allowance to specific loans ratio					43.05%
Balance of loans collectively evaluated for credit loss	$ 2,201,857,000		$ 2,201,857,000
Balance of loans collectively evaluated					$ 789,613,000
Allowance related to loans evaluated collectively	$ 53,148,000		$ 53,148,000
Allowance for loans collectively evaluated					$ 7,578,000
Collective allowance to collective loans ratio	2.41%		2.41%		0.96%
Commercial [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance of loans acquired					$ 2,539,000
Allowance for loans acquired					$ 0
Allowance to loans acquired with deteriorated credity quality ratio					0.00%
Commercial [Member] | Initial allowance on purchased credit deteriorated loans at adoption of ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 1,549,000
Balance at end of period					$ 1,549,000
Commercial [Member] | Transition impact of adopting ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			2,988,000
Balance at end of period					2,988,000
Consumer [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			2,086,000	2,113,000	2,113,000
Initial allowance on acquired Revere PCD loans			87,000
Provision for credit losses			3,460,000		565,000
Charge-offs			(286,000)		(783,000)
Recoveries			93,000		191,000
Net recoveries (charge-offs)			(193,000)		(592,000)
Balance at end of period	$ 6,359,000		6,359,000		2,086,000
Total loans	$ 558,434,000		$ 558,434,000		$ 466,764,000
Allowance for loan losses to total loans ratio	1.14%		1.14%		0.45%
Balance of loans individually evaluated for credit loss	$ 1,282,000		$ 1,282,000
Allowance related to loans evaluated individually	$ 99,000		$ 99,000
Individual allowance to loans evaluated individually ratio	7.72%		7.72%
Balance of loans collectively evaluated for credit loss	$ 557,152,000		$ 557,152,000
Balance of loans collectively evaluated					$ 465,771,000
Allowance related to loans evaluated collectively	$ 6,260,000		$ 6,260,000
Allowance for loans collectively evaluated					$ 2,086,000
Collective allowance to collective loans ratio	1.12%		1.12%		0.45%
Consumer [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance of loans acquired					$ 993,000
Allowance for loans acquired					$ 0
Allowance to loans acquired with deteriorated credity quality ratio					0.00%
Consumer [Member] | Initial allowance on purchased credit deteriorated loans at adoption of ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 99,000
Balance at end of period					$ 99,000
Consumer [Member] | Transition impact of adopting ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			820,000
Balance at end of period					820,000
Commercial Real Estate Portfolio Segment | Commercial AD&C [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			7,590,000	5,944,000	5,944,000
Initial allowance on acquired Revere PCD loans			1,248,000
Provision for credit losses			7,604,000		1,418,000
Charge-offs			0		0
Recoveries			0		228,000
Net recoveries (charge-offs)			0		228,000
Balance at end of period	$ 19,018,000		19,018,000		7,590,000
Total loans	$ 997,423,000		$ 997,423,000		$ 684,010,000
Allowance for loan losses to total loans ratio	1.91%		1.91%		1.11%
Balance of loans individually evaluated for credit loss	$ 2,957,000		$ 2,957,000
Balance of loans specifically evaluated for impairment					$ 829,000
Allowance related to loans evaluated individually	$ 635,000		$ 635,000
Allowance for loans specifically evaluated for impairment					$ 132,000
Individual allowance to loans evaluated individually ratio	21.47%		21.47%
Specific allowance to specific loans ratio					15.92%
Balance of loans collectively evaluated for credit loss	$ 994,466,000		$ 994,466,000
Balance of loans collectively evaluated					$ 683,181,000
Allowance related to loans evaluated collectively	$ 18,383,000		$ 18,383,000
Allowance for loans collectively evaluated					$ 7,458,000
Collective allowance to collective loans ratio	1.85%		1.85%		1.09%
Commercial Real Estate Portfolio Segment | Commercial AD&C [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance of loans acquired					$ 0
Allowance for loans acquired					$ 0
Allowance to loans acquired with deteriorated credity quality ratio					0.00%
Commercial Real Estate Portfolio Segment | Commercial AD&C [Member] | Initial allowance on purchased credit deteriorated loans at adoption of ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 0
Balance at end of period					$ 0
Commercial Real Estate Portfolio Segment | Commercial AD&C [Member] | Transition impact of adopting ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			2,576,000
Balance at end of period					2,576,000
Commercial Real Estate Portfolio Segment | Commercial Investor RE [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			18,407,000	17,603,000	17,603,000
Initial allowance on acquired Revere PCD loans			7,973,000
Provision for credit losses			22,567,000		788,000
Charge-offs			0		0
Recoveries			4,000		16,000
Net recoveries (charge-offs)			4,000		16,000
Balance at end of period	$ 46,940,000		46,940,000		18,407,000
Total loans	$ 3,581,778,000		$ 3,581,778,000		$ 2,169,156,000
Allowance for loan losses to total loans ratio	1.31%		1.31%		0.85%
Balance of loans individually evaluated for credit loss	$ 27,257,000		$ 27,257,000
Balance of loans specifically evaluated for impairment					$ 9,212,000
Allowance related to loans evaluated individually	$ 2,573,000		$ 2,573,000
Allowance for loans specifically evaluated for impairment					$ 1,529,000
Individual allowance to loans evaluated individually ratio	9.44%		9.44%
Specific allowance to specific loans ratio					16.60%
Balance of loans collectively evaluated for credit loss	$ 3,554,521,000		$ 3,554,521,000
Balance of loans collectively evaluated					$ 2,150,400,000
Allowance related to loans evaluated collectively	$ 44,367,000		$ 44,367,000
Allowance for loans collectively evaluated					$ 16,878,000
Collective allowance to collective loans ratio	1.25%		1.25%		0.78%
Commercial Real Estate Portfolio Segment | Commercial Investor RE [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance of loans acquired					$ 9,544,000
Allowance for loans acquired					$ 0
Allowance to loans acquired with deteriorated credity quality ratio					0.00%
Commercial Real Estate Portfolio Segment | Commercial Investor RE [Member] | Initial allowance on purchased credit deteriorated loans at adoption of ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 1,114,000
Balance at end of period					$ 1,114,000
Commercial Real Estate Portfolio Segment | Commercial Investor RE [Member] | Transition impact of adopting ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			(3,125,000)
Balance at end of period					(3,125,000)
Commercial Real Estate Portfolio Segment | Commercial Owner Occupied RE [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			6,884,000	6,307,000	6,307,000
Initial allowance on acquired Revere PCD loans			2,782,000
Provision for credit losses			8,627,000		577,000
Charge-offs			0		0
Recoveries			0		0
Net recoveries (charge-offs)			0		0
Balance at end of period	$ 18,680,000		18,680,000		6,884,000
Total loans	$ 1,601,803,000		$ 1,601,803,000		$ 1,288,677,000
Allowance for loan losses to total loans ratio	1.17%		1.17%		0.53%
Balance of loans individually evaluated for credit loss	$ 6,729,000		$ 6,729,000
Balance of loans specifically evaluated for impairment					$ 4,148,000
Allowance related to loans evaluated individually	$ 32,000		$ 32,000
Allowance for loans specifically evaluated for impairment					$ 23,000
Individual allowance to loans evaluated individually ratio	0.48%		0.48%
Specific allowance to specific loans ratio					0.55%
Balance of loans collectively evaluated for credit loss	$ 1,595,074,000		$ 1,595,074,000
Balance of loans collectively evaluated					$ 1,284,529,000
Allowance related to loans evaluated collectively	$ 18,648,000		$ 18,648,000
Allowance for loans collectively evaluated					$ 6,861,000
Collective allowance to collective loans ratio	1.17%		1.17%		0.53%
Commercial Real Estate Portfolio Segment | Commercial Owner Occupied RE [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance of loans acquired					$ 0
Allowance for loans acquired					$ 0
Allowance to loans acquired with deteriorated credity quality ratio					0.00%
Commercial Real Estate Portfolio Segment | Commercial Owner Occupied RE [Member] | Initial allowance on purchased credit deteriorated loans at adoption of ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 0
Balance at end of period					$ 0
Commercial Real Estate Portfolio Segment | Commercial Owner Occupied RE [Member] | Transition impact of adopting ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			387,000
Balance at end of period					387,000
Residential Real Estate Portfolio Segment | Residential Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			8,803,000	8,881,000	8,881,000
Initial allowance on acquired Revere PCD loans			243,000
Provision for credit losses			4,166,000		474,000
Charge-offs			(414,000)		(690,000)
Recoveries			66,000		138,000
Net recoveries (charge-offs)			(348,000)		(552,000)
Balance at end of period	$ 12,476,000		12,476,000		8,803,000
Total loans	$ 1,211,745,000		$ 1,211,745,000		$ 1,149,327,000
Allowance for loan losses to total loans ratio	1.03%		1.03%		0.77%
Balance of loans individually evaluated for credit loss	$ 1,350,000		$ 1,350,000
Balance of loans specifically evaluated for impairment					$ 1,717,000
Allowance related to loans evaluated individually	$ 0		$ 0
Allowance for loans specifically evaluated for impairment					$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Specific allowance to specific loans ratio					0.00%
Balance of loans collectively evaluated for credit loss	$ 1,210,395,000		$ 1,210,395,000
Balance of loans collectively evaluated					$ 1,147,602,000
Allowance related to loans evaluated collectively	$ 12,476,000		$ 12,476,000
Allowance for loans collectively evaluated					$ 8,803,000
Collective allowance to collective loans ratio	1.03%		1.03%		0.77%
Residential Real Estate Portfolio Segment | Residential Mortgage | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance of loans acquired					$ 8,000
Allowance for loans acquired					$ 0
Allowance to loans acquired with deteriorated credity quality ratio					0.00%
Residential Real Estate Portfolio Segment | Residential Mortgage | Initial allowance on purchased credit deteriorated loans at adoption of ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 0
Balance at end of period					$ 0
Residential Real Estate Portfolio Segment | Residential Mortgage | Transition impact of adopting ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			(388,000)
Balance at end of period					(388,000)
Residential Real Estate Portfolio Segment | Residential Construction
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			967,000	$ 1,261,000	1,261,000
Initial allowance on acquired Revere PCD loans			6,000
Provision for credit losses			671,000		(302,000)
Charge-offs			0		0
Recoveries			3,000		8,000
Net recoveries (charge-offs)			3,000		8,000
Balance at end of period	$ 1,372,000		1,372,000		967,000
Total loans	$ 169,050,000		$ 169,050,000		$ 146,279,000
Allowance for loan losses to total loans ratio	0.81%		0.81%		0.66%
Balance of loans individually evaluated for credit loss	$ 0		$ 0
Balance of loans specifically evaluated for impairment					$ 0
Allowance related to loans evaluated individually	$ 0		$ 0
Allowance for loans specifically evaluated for impairment					$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Specific allowance to specific loans ratio					0.00%
Balance of loans collectively evaluated for credit loss	$ 169,050,000		$ 169,050,000
Balance of loans collectively evaluated					$ 146,279,000
Allowance related to loans evaluated collectively	$ 1,372,000		$ 1,372,000
Allowance for loans collectively evaluated					$ 967,000
Collective allowance to collective loans ratio	0.81%		0.81%		0.66%
Residential Real Estate Portfolio Segment | Residential Construction | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance of loans acquired					$ 0
Allowance for loans acquired					$ 0
Allowance to loans acquired with deteriorated credity quality ratio					0.00%
Residential Real Estate Portfolio Segment | Residential Construction | Initial allowance on purchased credit deteriorated loans at adoption of ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ 0
Balance at end of period					$ 0
Residential Real Estate Portfolio Segment | Residential Construction | Transition impact of adopting ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period			$ (275,000)
Balance at end of period					$ (275,000)