SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)	1 Months Ended	3 Months Ended			9 Months Ended
	May 31, 2020 shares	Sep. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2021 USD ($) segment $ / shares shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2020 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | segment					2
Federal funds sales periods					1 day
Interest-earning deposits maturities period					1 year
Servicing rights					$ 0
Amount of loans serviced		$ 91,400,000			$ 91,400,000		$ 65,500,000
Threshold period for loan					30 days
Threshold for determining nonaccrual status		90 days			90 days
Loan modifications					$ 947,000,000.0
Post-modification outstanding recorded investment		$ 455,000			3,471,000	$ 427,000
Sale of loans servicing released to third party			$ 56,500,000	$ 355,000,000.0	$ 124,000,000
Closing stock price of common stock | $ / shares		$ 33.36			$ 33.36
Stock options granted | shares		0			0
Issuance of shares for Employee Stock Purchase Plan, shares | shares		6,889		5,598	21,661	52,533
Antidilutive securities | shares				374,366	261,610	207,466
Restricted stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Shares granted | shares		0			211,356
Compensation cost		$ 1,800,000		$ 1,800,000	$ 5,300,000	$ 5,100,000
Restricted stock [Member] | Tranche One [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock option vesting term					3 years
Restricted stock [Member] | Tranche Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock option vesting term					5 years
Performance Shares [Member]
Summary Of Significant Accounting Policies [Line Items]
Shares granted | shares		0			51,710
Restricted Stock Units [Member]
Summary Of Significant Accounting Policies [Line Items]
Unrecognized compensation cost		$ 14,300,000			$ 14,300,000
Weighted average period over which unrecognized compensation is expected to be recognized (in years)					1 year 6 months 7 days
Employee Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Compensation cost		$ 37,000		$ 9,000	$ 93,000	$ 199,000
Number of share purchase rights authorized | shares		150,000			150,000
Additional common stock to be issued | shares	200,000
Share purchase rights offering period					12 months
Percentage of closing market price on purchase date					85.00%
Deferral [Member]
Summary Of Significant Accounting Policies [Line Items]
Post-modification outstanding recorded investment					$ 33,074,000
U.S. Small Business Administration (SBA) Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Paycheck Protection Program, maximum credit facility		$ 650,000,000			650,000,000
Paycheck protection program loans remaining balance		$ 48,700,000			$ 48,700,000
Substandard [Member]
Summary Of Significant Accounting Policies [Line Items]
Loan reserve multiple		3.25			3.25		3.25
Special Mention [Member]
Summary Of Significant Accounting Policies [Line Items]
Loan reserve multiple		2.25			2.25		2.25
Consumer and Other [Member] | Closed-end Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Threshold for determining nonaccrual status		120 days			120 days
Consumer and Other [Member] | Open-end Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Threshold for determining nonaccrual status		180 days			180 days
Small Business Administration Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Loans held for sale		$ 11,600,000			$ 11,600,000		$ 6,000,000.0
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash equivalents original maturities period					90 days
Short term borrowings original maturities period					90 days
Finite-Lived Intangible Asset, Useful Life					15 years
Maximum [Member] | Employee Stock Option [Member]
Summary Of Significant Accounting Policies [Line Items]
Option term					10 years
Stock option vesting term					5 years
Maximum [Member] | Employee Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of compensation contributable					15.00%
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Finite-Lived Intangible Asset, Useful Life					5 years
Minimum [Member] | Employee Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of compensation contributable					1.00%
Peapack-Gladstone Mortgage Group, Inc. [Member] | Peapack Ventures, LLC [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of interest owned		99.00%			99.00%
Peapack-Gladstone Mortgage Group, Inc. [Member] | Peapack-Gladstone Realty, Inc [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of interest owned		79.00%			79.00%
PGB Trust & Investments of Delaware [Member] | Peapack Ventures, LLC [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of interest owned		1.00%			1.00%
Peapack Ventures, LLC [Member] | Peapack-Gladstone Realty, Inc [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of interest owned		21.00%			21.00%