Convertible Notes - Debt issuance costs and interest expense (Details) - Convertible Notes - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Interest expense related to the Convertible Notes
Contractual interest expense	$ 503	$ 503	$ 1,509	$ 1,509
Amortization of debt issuance costs	227	213	671	632
Amortization of debt discount	2,235	2,079	6,587	6,127
Total	$ 2,965	$ 2,795	$ 8,767	$ 8,268
Effective interest rate of the liability component	7.10%	7.10%	7.10%	7.10%
Amortization and interest expense capitalized	$ 1,180	$ 740	$ 2,334	$ 1,948