CONVERTIBLE NOTES - Unamortized discount and debt issuance costs (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Notes
Convertible notes
Unamortized discount	$ 667,228	$ 342,463
Debt issuance costs, gross	31,015	16,481
2025 Convertible notes
Convertible notes
Unamortized discount	64,721	89,123
Debt issuance costs, gross	4,495	6,230
2027 Convertible notes
Convertible notes
Unamortized discount	246,362	253,340
Debt issuance costs, gross	9,899	10,251
2028 Convertible Notes
Convertible notes
Unamortized discount	356,145	0
Debt issuance costs, gross	$ 16,621	$ 0