CONVERTIBLE NOTES - Unamortized discount and debt issuance costs (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Notes
Convertible notes
Unamortized discount	$ 628,820	$ 342,463
Debt issuance costs, gross	28,762	16,481
2028 Convertible notes
Convertible notes
Unamortized discount	337,968	0
Debt issuance costs, gross	15,569	0
2027 Convertible notes
Convertible notes
Unamortized discount	232,038	253,340
Debt issuance costs, gross	9,168	10,251
2025 Convertible notes
Convertible notes
Unamortized discount	58,814	89,123
Debt issuance costs, gross	$ 4,025	$ 6,230