Equity Method Investments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Nov. 09, 2020 AUD ($)	Sep. 30, 2020 AUD ($)	Dec. 31, 2019 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 54,700,000		$ 54,700,000				$ 56,900,000
(Loss) income from equity method investments	$ (10,600,000)	$ (20,700,000)	(25,700,000)	$ (7,500,000)
Cash receipts from Middlemount Coal Pty			0	14,700,000
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Cash receipts from Middlemount Coal Pty			$ 0	14,700,000
Equity interest percentage of revolving loans limit	50.00%		50.00%			50.00%
Revolving loan limit						$ 120,000,000
Financing receivable, stated interest rate (in percent)			10.00%
Intercompany loans, carrying value	$ 42,600,000		$ 42,600,000				$ 17,500,000
Middlemount Mine | Subsequent Event
Schedule of Equity Method Investments [Line Items]
Revolving loan limit					$ 160,000,000
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
(Loss) income from equity method investments	$ (10,600,000)	$ (18,800,000)	$ (25,700,000)	$ (5,300,000)