Intangible Contract Assets and Liabilities - Textual (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Coal Supply Agreements
Asset impairment	$ 1,487,400,000	$ 270,200,000	$ 0
Take-or-pay contracts
Coal Supply Agreements
Presented periods amortization	8,200,000	16,600,000	26,600,000
Take-or-Pay Contracts
Presented periods amortization	8,200,000	16,600,000	26,600,000
2021	4,000,000
2022	3,000,000
2023	3,000,000
2024	4,000,000
2025	1,000,000
Total thereafter	20,000,000
Coal supply agreements
Coal Supply Agreements
Presented periods amortization	4,200,000	23,200,000	93,000,000.0
Asset impairment	4,500,000
2021	1,000,000
2022	2,000,000
2023	2,000,000
2024	2,000,000
2025	3,000,000
Take-or-Pay Contracts
Presented periods amortization	$ 4,200,000	$ 23,200,000	$ 93,000,000.0

Finite-Lived Intangible Liabilities, Amortization Expense, Beyond Five Years

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Finite-Lived Intangible Liabilities, Amortization Expense, Next Twelve Months

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Finite-Lived Intangible Liabilities, Amortization Expense, Year Five

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Finite-Lived Intangible Liabilities, Amortization Expense, Year Four

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Finite-Lived Intangible Liabilities, Amortization Expense, Year Three

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Finite-Lived Intangible Liabilities, Amortization Expense, Year Two

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Finite-Lived Intangible Liabilities, Net, Amortization Expense, Fiscal Year Maturity [Abstract]

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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