Financial Instruments and Other Guarantees (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Nov. 06, 2020
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount	$ 443,200,000		$ 443,200,000
Surety bonds amount	1,462,400,000		1,462,400,000
Asset retirement obligations	710,200,000		710,200,000
Surety bonds issued to support asset retirement obligations	1,293,400,000		1,293,400,000
Letters of credit issued to support asset retirement obligations	316,100,000		316,100,000
Net interest expense	45,500,000	$ 34,900,000	143,300,000	$ 102,300,000
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
Debt instrument, transaction support agreements, fair value of second liens on mining equipment					200,000,000
Transaction support agreements, additional collateral to be posted per year through 2025					25,000,000
Transaction support agreements, additional collateral term, free cash flow in any twelve-month period					100,000,000
Transaction support agreements, additional collateral term, sale of assets benchmark (in excess of)					$ 10,000,000
Accounts Receivable Securitization Program, April 1, 2022 | Secured debt
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount	133,600,000		133,600,000
Line of credit facility, maximum borrowing capacity	250,000,000		250,000,000
Outstanding borrowings	0		0
Accounts Receivable from Securitization	12,600,000		12,600,000
Net interest expense	$ 1,000,000	$ 800,000	$ 3,000,000	$ 2,900,000
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%