SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)	1 Months Ended	3 Months Ended				6 Months Ended
	May 31, 2020 shares	Mar. 31, 2022 USD ($) segment $ / shares shares	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jan. 01, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020
Summary Of Significant Accounting Policies [Line Items]
Net decrease to retained earnings		$ (290,718,000)						$ (274,288,000)
Reduction to allowance for credit losses		$ (188,000)
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		$ 118,100,000						$ 97,500,000
Threshold period for loan		30 days
Threshold for determining nonaccrual status		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		$ 12,500,000
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		$ 34.75
Stock options granted | shares		0
Issuance of shares for Employee Stock Purchase Plan, shares | shares		6,808		8,425
Antidilutive securities | shares		299,433		239,472
Restricted stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Shares granted | shares		233,910
Compensation cost		$ 1,900,000		$ 1,700,000
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		71,482
Restricted Stock Units [Member]
Summary Of Significant Accounting Policies [Line Items]
Unrecognized compensation cost		$ 20,900,000
Weighted average period over which unrecognized compensation is expected to be recognized (in years)		1 year 6 months 25 days
Employee Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Compensation cost		$ 35,000		$ 30,000
Number of share purchase rights authorized | shares		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		$ 12,656,000
U.S. Small Business Administration (SBA) Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Paycheck Protection Program, maximum credit facility		650,000,000
Paycheck protection program loans remaining balance		$ 9,600,000
Consumer and Other [Member] | Closed-end Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Threshold for determining nonaccrual status		120 days
Consumer and Other [Member] | Open-end Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Threshold for determining nonaccrual status		180 days
Small Business Administration Loans [Member]
Summary Of Significant Accounting Policies [Line Items]
Loans held for sale		$ 26,400,000						$ 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 decrease 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%
Peapack-Gladstone Mortgage Group, Inc. [Member] | Peapack-Gladstone Realty, Inc [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of interest owned		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%
Peapack Ventures, LLC [Member] | Peapack-Gladstone Realty, Inc [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of interest owned		21.00%