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INDEBTEDNESS - Interest Expense on Convertible Notes (Details) - Convertible Debt - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	May 25, 2018	Mar. 06, 2017
Debt Instrument [Line Items]
Contractual interest expense	$ 1,277	$ 1,469	$ 3,831	$ 2,732
Amortization of debt discount and issuance costs	9,843	11,627	29,176	22,850
Total	$ 11,120	$ 13,096	$ 33,007	$ 25,582
2023 Notes
Debt Instrument [Line Items]
Effective interest rate of the liability component	4.69%		4.69%		4.69%
2022 Notes
Debt Instrument [Line Items]
Effective interest rate of the liability component	5.34%		5.34%			5.34%