Summary of Significant Accounting Policies (Details)				3 Months Ended						9 Months Ended		12 Months Ended
		Oct. 06, 2020 USD ($)	May 29, 2020	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reverse recapitalization, conversion ratio			0.112	0.112						0.112
Net loss				$ 29,114,000	$ 49,790,000	$ 34,618,000	$ 32,092,000	$ 21,947,000	$ 23,440,000	$ 113,522,000	$ 77,479,000
Net cash used in operating activities										85,212,000	44,095,000
Cash				11,505,000			4,963,000			11,505,000	4,963,000	$ 6,220,000
Working capital				42,700,000						42,700,000
Liabilities				205,967,000						205,967,000		573,888,000
Accumulated deficit	[1]			497,889,000						497,889,000		444,438,000
Cash equivalents				0						0		0
Restricted cash				39,654,000						39,654,000		42,747,000
Impairment on equipment not in use - Note 5				5,331,000			$ 0			5,331,000	$ 0
Asset impairment charges				0
Mineral properties				0						0		$ 0
Sprott Credit Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Debt, amount due in next 12 months				2,900,000						2,900,000
Subsequent event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from issuance of equity		$ 83,100,000
Held-for-sale
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Impairment on equipment not in use - Note 5				5,300,000						5,300,000
Assets held for sale, noncurrent				$ 2,300,000						$ 2,300,000
Non-employee director phantom stock plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock considered for cash payment of grants issued | shares										1		1	1	1	1

[1] Retroactively restated for the reverse recapitalization as described in Note 2 - Summary of Significant Accounting Policies.