REVENUES - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Capitalized Contract Cost [Line Items]
Capitalized contract cost, impairment loss	$ 0	$ 0	$ 0	$ 0
Fulfillment Costs And Commission Fees
Capitalized Contract Cost [Line Items]
Capitalized contract cost	24,200,000		24,200,000		$ 24,900,000
Amortization of capitalized contract costs	$ 2,500,000	$ 2,500,000	$ 7,300,000	$ 7,200,000