Company Overview - Schedule of prior period adjustments - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Assets	$ 220,025		$ 232,626	$ 192,681
Liabilities:
Current liabilities	31,081		21,681	21,947
Other liabilities, non-current - Note 8	1,298		1,650	135
Debt, net, non-current - Note 9	147,856		142,665	139,044
Royalty obligation, non-current - Note 10	29,432		29,839	29,699
Asset retirement obligation, non-current - Note 12	4,989		4,785	4,561
Warrant liabilities, non-current - Note 11				13,126
Total liabilities	219,617		216,009	208,512
Stockholders' (deficit) equity
Common stock, $0.0001 par value; 400,000,000 shares authorized; 59,977,318 issued and outstanding at June 30, 2021; and 59,901,306 issued and outstanding at December 31, 2020	6		6	5
Additional paid-in capital	539,276		537,370	453,861
Accumulated deficit	(538,874)		(520,759)	(469,697)
Total stockholders' equity	408	$ 7,540	16,617	(15,831)	$ (473,869)	$ (439,251)
Total liabilities and stockholders' equity	$ 220,025		$ 232,626	192,681
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Assets				192,681
Liabilities:
Current liabilities				21,947
Other liabilities, non-current - Note 8				153
Debt, net, non-current - Note 9				139,044
Royalty obligation, non-current - Note 10				29,699
Asset retirement obligation, non-current - Note 12				4,561
Warrant liabilities, non-current - Note 11				0
Total liabilities				195,404
Stockholders' (deficit) equity
Common stock, $0.0001 par value; 400,000,000 shares authorized; 59,977,318 issued and outstanding at June 30, 2021; and 59,901,306 issued and outstanding at December 31, 2020				5
Additional paid-in capital				466,047
Accumulated deficit				(468,775)
Total stockholders' equity				(2,723)
Total liabilities and stockholders' equity				192,681
Restatement Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Assets				0
Liabilities:
Current liabilities				0
Other liabilities, non-current - Note 8				(18)
Debt, net, non-current - Note 9				0
Royalty obligation, non-current - Note 10				0
Asset retirement obligation, non-current - Note 12				0
Warrant liabilities, non-current - Note 11				13,126
Total liabilities				13,108
Stockholders' (deficit) equity
Common stock, $0.0001 par value; 400,000,000 shares authorized; 59,977,318 issued and outstanding at June 30, 2021; and 59,901,306 issued and outstanding at December 31, 2020				0
Additional paid-in capital				(12,186)
Accumulated deficit				(922)
Total stockholders' equity				(13,108)
Total liabilities and stockholders' equity				$ 0