INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Tax credits	$ 700	$ 800		$ 600
Cash and cash equivalents	39,623	21,262	$ 39,623	$ 22,838
Cardlytics UK
Operating Loss Carryforwards [Line Items]
Cash and cash equivalents	3,800
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	249,800	220,500
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	90,500	76,300
Foreign
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 13,900	$ 11,400