Convertible Notes - Debt issuance costs and interest expense (Details) - Convertible Notes - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Interest expense related to the Convertible Notes
Contractual interest expense	$ 503	$ 503
Amortization of debt issuance costs	220	208
Amortization of debt discount	2,156	2,006
Total	$ 2,879	$ 2,717
Effective interest rate of the liability component	7.10%	7.10%
Amortization and interest expense capitalized	$ 650	$ 465