Convertible Notes - Debt issuance costs and interest expense (Details) - Convertible Notes - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Interest expense related to the Convertible Notes
Contractual interest expense	$ 503	$ 503	$ 1,006	$ 1,006
Amortization of debt issuance costs	224	211	444	419
Amortization of debt discount	2,196	2,042	4,352	4,048
Total	$ 2,923	$ 2,756	$ 5,802	$ 5,473
Effective interest rate of the liability component	7.10%	7.10%	7.10%	7.10%
Amortization and interest expense capitalized	$ 504	$ 743	$ 1,154	$ 1,208