Basis of Presentation - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Nov. 09, 2020
Debt Instrument [Line Items]
Percentage of decline of coal-fueled generation	24.00%
Percentage of coal-fueled generation out of overall generation mix	19.00%
Increase (decrease) in income (loss) from continuing operations, net of income taxes	$ (1,841,300,000)
Increase (decrease) in EBITDA	(487,700,000)
Borrowings from revolving credit facility and accounts receivable securitization program	360,000,000.0
Repayments of lines of credit	70,000,000.0
Letters of Credit Issued	134,200,000
Surety bonds amount	1,628,700,000
Letters of credit outstanding, amount	334,800,000
Secured debt
Debt Instrument [Line Items]
Increase (decrease) in accounts receivable from securitization program	$ (73,900,000)
Surety Bond | Subsequent Event
Debt Instrument [Line Items]
Transaction support agreements, additional collateral demands		$ 800,000,000
Transaction support agreements, percentage of Participating Sureties		99.00%
Transaction support agreements, additional collateral to be posted		$ 75,000,000.0
Transaction support agreements, minimum collateral to be posted in the form of letters of credit		40,000,000.0
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