Intangible Contract Assets and Liabilities - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Asset impairment	$ 0	$ 20,000,000.0	$ 1,418,100,000	$ 20,000,000.0
Contract-Based Intangible Assets
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Asset impairment			4,500,000
Coal supply agreements
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Presented periods amortization	400,000	5,300,000	4,100,000	16,900,000
Remainder of 2020	1,000,000		1,000,000
2021	1,000,000		1,000,000
2022	2,000,000		2,000,000
2023	2,000,000		2,000,000
2024	2,000,000		2,000,000
Thereafter	3,000,000		3,000,000
Finite-Lived Intangible Liabilities, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Presented periods amortization	400,000	5,300,000	4,100,000	16,900,000
Take-or-pay contracts
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Presented periods amortization	1,500,000	2,700,000	6,800,000	13,900,000
Finite-Lived Intangible Liabilities, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Presented periods amortization	1,500,000	$ 2,700,000	6,800,000	$ 13,900,000
Remainder of 2020	1,000,000		1,000,000
2021	4,000,000		4,000,000
2022	3,000,000		3,000,000
2023	3,000,000		3,000,000
2024	3,000,000		3,000,000
Thereafter	$ 19,000,000		$ 19,000,000