FAIR VALUE - Long-Term Debt and Convertible Notes (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Carrying Amount | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Construction loan	$ 24,844	$ 24,502
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Construction loan	24,844	24,502
2027 Convertible notes | Carrying Amount | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	469,595
2027 Convertible notes | Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	916,158
2025 Convertible notes | Carrying Amount | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	311,598	664,749
2025 Convertible notes | Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	$ 696,337	$ 956,196