DEBT - Summary of Interest Expense of Convertible Notes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Accretion of debt discount and debt issuance costs	$ 23,764	$ 21,877	$ 19,415
Convertible debt
Debt Instrument [Line Items]
Stated coupon interest	6,708	8,624	8,625
Accretion of debt discount and debt issuance costs	15,398	18,288	16,784
Total interest expense	22,106	$ 26,912	$ 25,409
Convertible Senior Notes, 5.0% | Convertible debt
Debt Instrument [Line Items]
Stated coupon interest	2,250
Accretion of debt discount and debt issuance costs	2,583
Total interest expense	$ 4,833