CONVERTIBLE NOTES - Unamortized discount and debt issuance costs (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Notes
Convertible notes
Unamortized discount	$ 648,281	$ 342,463
Debt issuance costs, gross	29,885	16,481
2028 Convertible notes
Convertible notes
Unamortized discount	347,191	0
Debt issuance costs, gross	16,098	0
2027 Convertible notes
Convertible notes
Unamortized discount	239,267	253,340
Debt issuance costs, gross	9,525	10,251
2025 Convertible notes
Convertible notes
Unamortized discount	61,823	89,123
Debt issuance costs, gross	$ 4,262	$ 6,230