SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)	1 Months Ended	3 Months Ended			6 Months Ended
	May 31, 2020 shares	Jun. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($) shares	Jun. 30, 2021 USD ($) segment $ / shares shares	Jun. 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		$ 83,100,000			$ 83,100,000		$ 65,500,000
Threshold period for loan					30 days
Threshold for determining nonaccrual status		90 days			90 days
Loan modifications					$ 946,800,000
Post-modification outstanding recorded investment		$ 3,139,000		$ 139,000	3,139,000	$ 436,000
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		$ 31.07			$ 31.07
Stock options granted | shares		0			0
Issuance of shares for Employee Stock Purchase Plan, shares | shares		6,347		46,935	14,772	46,935
Antidilutive securities | shares		13,374		297,320	262,423	713,789
Restricted stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Shares granted | shares		21,200			211,356
Compensation cost		$ 1,800,000		$ 1,700,000	$ 3,500,000	$ 3,300,000
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		$ 16,100,000			$ 16,100,000
Weighted average period over which unrecognized compensation is expected to be recognized (in years)					1 year 7 months 2 days
Employee Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Compensation cost		$ 26,000		$ 149,000	$ 56,000	$ 190,000
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					$ 36,700,000
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		$ 83,800,000			$ 83,800,000
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		$ 5,600,000			$ 5,600,000		$ 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%