Company Overview - Schedule of prior period adjustments - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Assets	$ 195,332			$ 232,626	$ 173,186
Liabilities:
Current liabilities	31,167			21,681	28,649
Other liabilities - Note 8	541			1,650	1,703
Debt, net - Note 9	146,189			142,665	140,959
Deferred gain on sale of royalty - Note 10	29,714			29,839	29,812
Asset retirement obligation - Note 12	5,091			4,785	4,654
Warrant liabilities - Note 11					16,461
Total liabilities	216,851			216,009	222,238
Stockholders' (deficit) equity
Common stock, $0.0001 par value; 400,000,000 shares authorized; 60,410,922 issued and outstanding at September 30, 2021; and 59,901,306 issued and outstanding at December 31, 2020	6			6	5
Additional paid-in capital	540,562			537,370	452,918
Accumulated deficit	(562,087)			(520,759)	(501,975)
Total stockholders' (deficit) equity	(21,519)	$ 408	$ 7,540	16,617	(49,052)	$ (15,831)	$ (473,869)	$ (439,251)
Total liabilities and stockholders' (deficit) equity	$ 195,332			$ 232,626	173,186
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Assets					173,186
Liabilities:
Current liabilities					28,649
Other liabilities - Note 8					1,893
Debt, net - Note 9					140,959
Deferred gain on sale of royalty - Note 10					29,812
Asset retirement obligation - Note 12					4,654
Warrant liabilities - Note 11					0
Total liabilities					205,967
Stockholders' (deficit) equity
Common stock, $0.0001 par value; 400,000,000 shares authorized; 60,410,922 issued and outstanding at September 30, 2021; and 59,901,306 issued and outstanding at December 31, 2020					5
Additional paid-in capital					465,103
Accumulated deficit					(497,889)
Total stockholders' (deficit) equity					(32,781)
Total liabilities and stockholders' (deficit) equity					173,186
Restatement Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Assets					0
Liabilities:
Current liabilities					0
Other liabilities - Note 8					(190)
Debt, net - Note 9					0
Deferred gain on sale of royalty - Note 10					0
Asset retirement obligation - Note 12					0
Warrant liabilities - Note 11					16,461
Total liabilities					16,271
Stockholders' (deficit) equity
Common stock, $0.0001 par value; 400,000,000 shares authorized; 60,410,922 issued and outstanding at September 30, 2021; and 59,901,306 issued and outstanding at December 31, 2020					0
Additional paid-in capital					(12,185)
Accumulated deficit					(4,086)
Total stockholders' (deficit) equity					(16,271)
Total liabilities and stockholders' (deficit) equity					$ 0