Equity Method Investments (Details)	3 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2021 AUD ($)	Dec. 31, 2020 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 21,200,000			$ 24,600,000
(Loss) income from equity method investments	(900,000)	$ (9,100,000)
Cash receipts from Middlemount Coal Pty	2,300,000	0
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Cash receipts from Middlemount Coal Pty	$ (2,300,000)	0
Equity interest percentage of revolving loans limit	50.00%		50.00%
Revolving loan limit			$ 160,000,000
Financing receivable, stated interest rate (in percent)	10.00%
Intercompany loans, carrying value	$ 43,300,000			$ 46,200,000
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
(Loss) income from equity method investments	$ (900,000)	$ (9,100,000)