FAIR VALUE - Long-Term Debt and Convertible Notes (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Carrying Amount | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Construction loan	$ 24,855	$ 24,502
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Construction loan	24,855	24,502
2027 Convertible notes | Carrying Amount | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	476,527
2027 Convertible notes | Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	810,514
2025 Convertible notes | Carrying Amount | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	315,643	664,749
2025 Convertible notes | Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	$ 586,332	$ 956,196