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 ($) shares	Jun. 30, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($) Derivative
Debt Instrument [Line Items]
Advances from the FHLB							$ 181,118,000	$ 172,000,000
Long-term debt prepayment fee							0	2,828,000	$ 0
Securities sold under agreement to repurchase maturities due							$ 0	$ 20,000,000
Repayments of long-term debt				$ 20,000,000
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							$ 57,700,000
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							5 years	4 years
Collateralized FHLB Advances | Weighted Average
Debt Instrument [Line Items]
Weighted average interest rate of advances							2.10%	1.69%
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%
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
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