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DEBT - Convertible Senior Notes (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Aug. 07, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Aug. 01, 2017 $ / shares
Debt
Value of shares repurchased and retired			$ 97,500	$ 78,094
Liability component
Outstanding principal balance		$ 447,234	677,234
Unamortized debt discount and issuance costs		(64,379)	$ (77,872)
Common Stock
Debt
Share Price \| $ / shares					$ 13.28
Number of shares repurchased and retired \| shares			7,430,000	7,201,000
Value of shares repurchased and retired			$ 75	$ 72
2025 Notes
Debt
Debt discount		67,300
Convertible senior notes \| 2025 Notes
Liability component
Outstanding principal balance		192,500	192,500
Unamortized debt discount and issuance costs		(61,766)	(68,342)
Net carrying amount		130,734	124,158
Equity component, net		65,361	65,361
Interest expense
Contractual interest expense		4,799	1,925
Amortization of debt issuance costs		505	189
Amortization of debt discount		6,071	2,268
Total interest and amortization expense		$ 11,375	$ 4,382
Convertible senior notes \| 2025 Notes \| Over-Allotment Option
Debt
Loan amount	$ 17,500
Convertible senior notes \| 2025 Notes \| Private Placement
Debt
Loan amount	$ 192,500
Interest rate (as a percent)	2.50%
Convertible senior notes \| 2025 Notes \| Common Stock
Debt
Conversion rate	57.9240
Conversion price (dollars per share) \| $ / shares	$ 17.26	$ 17.26
Conversion premium (as a percent)	30.00%
Number of shares repurchased and retired \| shares	1,317,771
Value of shares repurchased and retired	$ 17,500