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Financial Instruments and Other Guarantees (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Nov. 06, 2020
Guarantee Obligations [Line Items]            
Letters of credit outstanding, amount $ 435,400,000   $ 435,400,000      
Surety bonds amount 1,555,000,000.0   1,555,000,000.0      
Asset retirement obligations 741,800,000   741,800,000      
Surety bonds issued to support asset retirement obligations 1,379,000,000.0   1,379,000,000.0      
Letters of credit issued to support asset retirement obligations 310,500,000   310,500,000      
Net interest expense 45,400,000 $ 34,300,000 97,800,000 $ 67,400,000    
Surety Bond            
Guarantee Obligations [Line Items]            
Transaction support agreements, percentage of Participating Sureties           99.00%
Transaction support agreements, additional collateral to be posted           $ 75,000,000.0
Debt instrument, transaction support agreements, fair value of second liens on mining equipment           200,000,000.0
Transaction support agreements, additional collateral to be posted per year through 2025           25,000,000.0
Transaction support agreements, additional collateral term, free cash flow in any twelve-month period           100,000,000.0
Transaction support agreements, additional collateral term, sale of assets benchmark (in excess of)           $ 10,000,000.0
Accounts Receivable Securitization Program, April 1, 2022 | Secured debt            
Guarantee Obligations [Line Items]            
Letters of credit outstanding, amount 125,500,000   125,500,000      
Line of credit facility, maximum borrowing capacity 250,000,000.0   250,000,000.0      
Outstanding borrowings 0   0      
Accounts Receivable from Securitization 300,000   300,000      
Collateral posted 13,600,000   13,600,000   $ 0  
Net interest expense $ 700,000 $ 800,000 $ 1,400,000 $ 1,500,000    
Secured debt | Accounts Receivable Securitization Program, April 1, 2022 | London Interbank Offered Rate (LIBOR) Swap Rate            
Guarantee Obligations [Line Items]            
Basis spread on variable rate 1.50%