Acquisitions - Summary of Estimated Fair Value of Acquired Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Fair value of Somerset Hills stock options converted to Lakeland Bancorp stock options	$ 1,603
Recognized amounts of identifiable assets and liabilities assumed at fair value
Goodwill	110,381	87,111
Somerset Hills Bancorp [Member]
Business Acquisition [Line Items]
Lakeland Bancorp stock issued	57,419
Cash Payment	6,460
Fair value of Somerset Hills stock options converted to Lakeland Bancorp stock options	1,603
Total Consideration Paid	65,482
Recognized amounts of identifiable assets and liabilities assumed at fair value
Cash and cash equivalents	80,359
Securities available for sale	1,737
Securities held to maturity	8,686
Federal Home Loan Bank stock	493
Loans and leases	243,888
Loans held for sale	2,532
Premises and equipment	5,214
Identifiable intangible assets	2,712
Accrued interest receivable and other assets	10,270
Deposits	(311,801)
Other liabilities	(1,877)
Total identifiable assets	42,213
Goodwill	$ 23,269