Debt - Additional Information (Long Term Debt) (Detail)		1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	Aug. 03, 2015 USD ($)	May 31, 2007 USD ($) shares	Jun. 30, 2003 USD ($) shares	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2016 USD ($) Derivative
Debt Instrument [Line Items]
Advances from the FHLB						$ 172,011,000	$ 220,900,000
Long-term debt prepayment fee						2,828,000	0	$ 2,407,000
Securities sold under agreement to repurchase maturities due						20,000,000	40,000,000
Securities sold under agreement to repurchase maturities due current						20,000,000
Gain on extinguishment of debt						$ 0	$ 0	$ 1,830,000
Cash Flow Hedging | Interest Rate Swaps
Debt Instrument [Line Items]
Debt instrument, face amount									$ 30,000,000.0
Debt instrument, description of variable rate basis						3 month LIBOR
Number of derivatives | Derivative									2
Short Term and Long Term Mortgage Backed Securities Sold Under Agreements To Repurchase
Debt Instrument [Line Items]
Pledged securities						$ 73,700,000
Federal Home Loan Bank of New York
Debt Instrument [Line Items]
Repayments of debt				$ 34,000,000
Long-term debt prepayment fee				638,000
Federal Home Loan Bank of New York | Maximum
Debt Instrument [Line Items]
Maturity of advances from the FHLB						4 years	4 years
Collateralized FHLB Advances | Weighted Average
Debt Instrument [Line Items]
Weighted average interest rate of advances						1.69%	1.81%
Debt Securities Sold under Agreements to Repurchase
Debt Instrument [Line Items]
Repayments of debt				20,000,000
Long-term debt prepayment fee				$ 2,200,000
Weighted average interest rate of advances						2.25%	3.26%
Junior Subordinated Debt | Lakeland Bancorp Capital Trust IV
Debt Instrument [Line Items]
Debt instrument, face amount		$ 20,600,000
Debt instrument, maturity date							Aug. 31, 2037
Debt instrument, basis point		1.52%
Distribution rate on securities		6.61%
Distribution of securities years		5 years
Trust preferred securities, issued | shares		20,000
Trust preferred securities, face value		$ 1,000
Proceeds of trust preferred securities, Total		$ 20,000,000
Preferred securities redeemed						2037
Gain on extinguishment of debt	$ 1,800,000
Debentures extinguished	$ 10,000,000
Junior Subordinated Debt | Lakeland Bancorp Capital II
Debt Instrument [Line Items]
Debt instrument, face amount			$ 20,600,000
Debt instrument, maturity date						Jun. 30, 2033
Debt instrument, basis point			3.10%
Distribution rate on securities			5.71%
Distribution of securities years			5 years
Trust preferred securities, issued | shares			20,000
Trust preferred securities, face value			$ 1,000
Proceeds of trust preferred securities, Total			$ 20,000,000
Preferred securities redeemed						2033
Subordinated Debenture Offering | Fixed To Floating
Debt Instrument [Line Items]
Debt instrument, offering date						Sep. 30, 2016
Debt instrument, face amount					$ 75,000,000.0
Debt instrument, maturity date						Sep. 30, 2026
Debt instrument, interest rate					5.125%
Debt instrument, interest rate repricing date						Sep. 30, 2021
Debt issuance costs						$ 1,500,000
Subordinated Debenture Offering | Fixed To Floating | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis point					3.97%
Debt instrument, description of variable rate basis						Three-month LIBOR