Revenue Recognition - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Contract with customer, liability	$ 5,100,000,000
Accounts receivable, credit loss expense (reversal)		$ 0
Revenue	$ 691,400,000	651,300,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, percentage	48.00%
Revenue	$ 691,400,000	651,300,000
Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenue		17,900,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue		17,900,000
Coal [Member]
Disaggregation of Revenue [Line Items]
Revenue	1,039,000,000	656,000,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue	1,039,000,000	656,000,000
Designated as Hedging Instrument
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Unrealized gain (loss) recognized in income on derivatives	(286,900,000)	(12,500,000)
Unrealized gain (loss) recognized in income on derivatives	(286,900,000)	(12,500,000)
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue	(335,400,000)	9,700,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue	(335,400,000)	9,700,000
Corporate and Other (1) | Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenue	19,000,000	17,900,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue	19,000,000	17,900,000
Coal Contract and Physical commodity purchase / sale contracts | Designated as Hedging Instrument
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Unrealized gain (loss) recognized in income on derivatives	(290,200,000)	(4,900,000)
Unrealized gain (loss) recognized in income on derivatives	$ (290,200,000)	$ (4,900,000)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, period	12 months
Trade receivables
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Accounts receivable, allowance for credit loss	$ 0		$ 0
Accounts receivable, allowance for credit loss	0		0
Miscellaneous Receivables
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Accounts receivable, allowance for credit loss	0		0
Accounts receivable, allowance for credit loss	$ 0		$ 0