Balance Sheet Items - Schedule of Property and equipment, net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 164,952		$ 164,952		$ 172,938
Less: accumulated depreciation	(100,183)		(100,183)		(87,644)
Property and equipment, net	64,769		64,769		85,294
Depreciation	4,818	$ 3,304	13,939	$ 13,636
Plant, machinery and equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	79,781		79,781		75,578
Structural fill site improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	55,760		55,760		55,760
Vehicles
Property, Plant and Equipment [Line Items]
Total property and equipment	18,396		18,396		19,163
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	2,847		2,847		2,741
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	262		262		262
Structural fill sites
Property, Plant and Equipment [Line Items]
Total property and equipment	432		432		7,110
Construction in progress
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 7,474		$ 7,474		$ 12,324