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Supplemental Financial Information - Narrative (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended
Nov. 30, 2014
Mar. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]    
Mine reserves, amortization period   21 years
Highview Enterprises Limited    
Schedule of Equity Method Investments [Line Items]    
Ownership interest, percent 8.00%  
Payments to acquire investments $ 2.8