SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)	1 Months Ended	3 Months Ended				6 Months Ended	9 Months Ended
	May 31, 2020 shares	Sep. 30, 2022 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) shares	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) Segment $ / shares shares	Sep. 30, 2021 USD ($) shares	Jan. 01, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020
Summary Of Significant Accounting Policies [Line Items]
Net increase to retained earnings		$ 329,116,000					$ 329,116,000			$ 274,288,000
Reduction to allowance for credit losses		(925,000)					$ (925,000)
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		$ 145,600,000					$ 145,600,000			$ 97,500,000
Threshold period for loan							30 days
Threshold for determining nonaccrual status		90 days					90 days
Non-impaired special mention loans multiple										2.25
Non-impaired substandard loans multiple											3.25
Loan modifications							$ 947,000,000.0
Post-modification outstanding recorded investment			$ 455,000				$ 12,429,000	$ 3,471,000
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		$ 33.65					$ 33.65
Stock options granted | shares		0					0
Issuance of shares for Employee Stock Purchase Plan, shares | shares		6,441	6,889				20,369	21,661
Antidilutive securities | shares		112,241					296,687	261,610
Goodwill impairment							$ 0
Restricted stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Shares granted | shares		0					235,534
Compensation cost		$ 2,100,000	$ 1,800,000				$ 5,700,000	$ 5,300,000
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					71,482
Restricted Stock Units [Member]
Summary Of Significant Accounting Policies [Line Items]
Unrecognized compensation cost		$ 17,000,000.0					$ 17,000,000.0
Weighted average period over which unrecognized compensation is expected to be recognized (in years)							1 year 4 months 20 days
Employee Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Compensation cost		$ 37,000	$ 37,000				$ 114,000	$ 93,000
Number of share purchase rights authorized | shares		150,000					150,000
Additional common stock to be issued | shares	200,000
Percentage of closing market price on purchase date							85.00%
Deferral [Member]
Summary Of Significant Accounting Policies [Line Items]
Post-modification outstanding recorded investment							$ 0
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		$ 1,900,000					$ 1,900,000
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		$ 10,100,000					$ 10,100,000			$ 32,500,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash equivalents original maturities period							90 days
Short term borrowings original maturities period							90 days
Paycheck Protection Loans Repayment Period							5 years
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]
Paycheck Protection Loans Repayment Period							2 years
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%
ASU 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Net increase to retained earnings									$ 3,900,000
Reduction to allowance for credit losses									$ 5,500,000
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%