MEZZANINE EQUITY - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended			12 Months Ended
	Feb. 13, 2019	Jan. 02, 2019	Nov. 08, 2018	Feb. 13, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Preferred stock, shares authorized (in shares)								60,000,000.0
Preferred stock, par value (in dollars per share)								$ 0.001
Gain on forward contracts for the issuance of convertible preferred stock					$ (16,800)	$ 88,900		$ (16,787)	$ 88,942	$ 0
Temporary equity, stock issued during period, value							$ 98,804	184,746
Temporary equity, accreted dividends								19,800
Cumulative undeclared convertible preferred stock dividend								$ 18,810	$ 957	$ 0
SPA Amendment, Initial Issuance
Temporary Equity [Line Items]
Convertible preferred stock issued (in shares)			100,000
Sale of stock, consideration received on transaction			$ 100,000
SPA Amendment, First Subsequent Issuance
Temporary Equity [Line Items]
Convertible preferred stock issued (in shares)		50,000
Sale of stock, consideration received on transaction		$ 50,000
SPA Amendment, Second Subsequent Issuance
Temporary Equity [Line Items]
Convertible preferred stock issued (in shares)	149,950
Sale of stock, consideration received on transaction	$ 150,000
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Preferred stock, par value (in dollars per share)						$ 1,000		$ 1,000	$ 1,000
Convertible preferred stock, shares issued (in shares)						100,000		299,950	100,000
Temporary equity, stock issued during period, value								$ 300,000	$ 100,000
Redeemable Convertible Preferred Stock | SPA Amendment, Private Placement
Temporary Equity [Line Items]
Convertible preferred stock issued (in shares)				299,950
Price per share (in dollars per share)				$ 1,000
Sale of stock, consideration received on transaction				$ 300,000
Sale of stock, number of converted shares (in shares)				56,100,000
Initial conversion price (in dollars per share)				$ 5.35
Series A Preferred Stock
Temporary Equity [Line Items]
Preferred stock, par value (in dollars per share)								$ 1,000
Preferred stock, dividend rate, percentage								6.50%