BUSINESS COMBINATIONS - Summary of Fair Value of Consideration Transferred in Acquisition (Details) - USD ($) $ in Thousands			8 Months Ended	12 Months Ended
		Apr. 30, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Payments for (Proceeds from) Previous Acquisition				$ 8,391	$ 0	$ 0
Contingent consideration liability incurred in connection with acquisition of business				1,589	$ 0	$ 0
Cloud Savings [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		$ 6,244
Cash			$ 64,363
Financing obligation incurred in connection with acquisition of business		8,600
Contingent consideration liability incurred in connection with acquisition of business		1,600	1,589
Total acquisition price		74,556		74,556
Accounts receivable		5,885
Prepaid expenses and other current assets		804
Property, equipment and software		226
Goodwill		46,515	$ 46,515	$ 46,515
Other intangible assets		83,841
Accounts payable		693
Accrued merchant and supplier payables		386
Accrued expenses and other current liabilities		6,130
Other non-current liabilities		2,076
Total liabilities assumed		9,285
Other Intangible Assets | Cloud Savings [Member]
Business Acquisition [Line Items]
Intangible assets acquired	[1]	687
Trade names | Cloud Savings [Member]
Business Acquisition [Line Items]
Intangible assets acquired	[1]	2,609
Developed technology | Cloud Savings [Member]
Business Acquisition [Line Items]
Intangible assets acquired	[1]	549
Merchant relationships | Cloud Savings [Member]
Business Acquisition [Line Items]
Intangible assets acquired	[1]	$ 20,322

[1] The estimated useful lives of the acquired intangible assets are 6 years for merchant relationships, 8 years for trade names, 2 years for developed technology, and 1 year for other intangible assets.