Balance Sheet Items - Schedule of Property and equipment, net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 171,745		$ 171,745		$ 172,938
Less: accumulated depreciation	(96,590)		(96,590)		(87,644)
Property and equipment, net	75,155		75,155		85,294
Depreciation	4,692	$ 3,285	9,121	$ 10,332
Plant, machinery and equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	79,046		79,046		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	19,263		19,263		19,163
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	2,785		2,785		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	7,110		7,110		7,110
Construction in progress
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 7,519		$ 7,519		$ 12,324