Debt - Additional Information (Long Term Debt) (Detail) - USD ($)					1 Months Ended		12 Months Ended
	Sep. 15, 2026	Jan. 06, 2022	Sep. 30, 2021	Aug. 03, 2015	May 31, 2007	Jun. 30, 2003	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Sep. 15, 2021	Sep. 30, 2016	Oct. 31, 2015	May 31, 2014
Debt Instrument [Line Items]
Advances from the FHLB							$ 325,000,000	$ 25,000,000
Subordinated debt							0	14,734,000	$ 0
Long-term debt extinguishment costs			$ 831,000				$ 0	$ 0	$ 831,000
Collateralized FHLB Advances | Weighted Average
Debt Instrument [Line Items]
Weighted average interest rate of advances							4.71%	0.77%
Subordinated Debenture Offering | Fixed To Floating, Due September 15, 2031
Debt Instrument [Line Items]
Debt instrument, face amount										$ 150,000,000
Debt instrument, interest rate										2.875%
Debt issuance costs										$ 2,300,000
Subordinated Debenture Offering | Fixed To Floating
Debt Instrument [Line Items]
Debt instrument, face amount											$ 75,000,000
Debt instrument, interest rate											5.125%
Debt issuance costs											$ 1,500,000
Subordinated Debenture Offering | Fixed To Floating Due June 15, 2036
Debt Instrument [Line Items]
Debt instrument, face amount		$ 18,000,000
Subordinated debt		$ 14,700,000
Subordinated Debenture Offering | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Fixed To Floating, Due September 15, 2031
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.65%
Subordinated Debenture Offering | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Fixed To Floating, Due September 15, 2031 | Forecast
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.20%
Subordinated Debenture Offering | London Interbank Offered Rate (LIBOR) 1 | Fixed To Floating
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			3.97%
Junior Subordinated Debt | Lakeland Bancorp Capital Trust IV
Debt Instrument [Line Items]
Debt instrument, face amount					$ 20,600,000
Long-term debt extinguishment costs				$ (1,800,000)
Distribution rate on securities					6.61%
Distribution of securities years					5 years
Trust preferred securities, issued (in shares)					20,000
Trust preferred securities, face value					$ 1,000
Proceeds of trust preferred securities					$ 20,000,000
Debentures extinguished				$ 10,000,000
Junior Subordinated Debt | Lakeland Bancorp Capital II
Debt Instrument [Line Items]
Debt instrument, face amount						$ 20,600,000
Distribution rate on securities						5.71%
Distribution of securities years						5 years
Trust preferred securities, issued (in shares)						20,000
Trust preferred securities, face value						$ 1,000
Proceeds of trust preferred securities						$ 20,000,000
Junior Subordinated Debt | Floating Rate Junior Subordinated Debentures Due June 15, 2036
Debt Instrument [Line Items]
Debt instrument, face amount		$ 18,600,000
Junior Subordinated Debt | London Interbank Offered Rate (LIBOR) 1 | Lakeland Bancorp Capital Trust IV
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate					1.52%
Junior Subordinated Debt | London Interbank Offered Rate (LIBOR) 1 | Lakeland Bancorp Capital II
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						3.10%
Constitution Capital Trust II
Debt Instrument [Line Items]
Net assets acquired		$ 557,000
Highlands Bancorp, Inc. | Subordinated Debenture Offering | Fixed Rate
Debt Instrument [Line Items]
Debt instrument, face amount												$ 7,500,000	$ 5,000,000
Debt instrument, interest rate												6.94%	8.00%