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Balance Sheet Items - Asset Retirement Obligation Narrative (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
USD ($)
site
Sep. 30, 2019
USD ($)
Sep. 30, 2020
USD ($)
site
Sep. 30, 2019
USD ($)
Jun. 30, 2020
USD ($)
Dec. 31, 2019
USD ($)
Jun. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]                
Number of structural fill sites with maintenance and monitoring subsequent to closure | site 2   2          
Asset retirement obligation $ 6,947 $ 17,767 $ 6,947 $ 17,767 $ 10,948 $ 15,131 $ 20,945 $ 26,065
Change in estimated cash flows 2,127 $ 0 $ 2,127 $ 0        
Asset retirement obligation, period decrease 279              
Costs incurred, asset retirement obligation incurred $ 1,848