Equity Method Investments (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 AUD ($)	Dec. 31, 2020 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 17,500,000		$ 17,500,000			$ 24,600,000
(Loss) income from equity method investments	$ (3,500,000)	$ (6,000,000.0)	(4,400,000)	$ (15,100,000)
Cash receipts from Middlemount Coal Pty Ltd and other related parties			2,600,000	0
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Cash receipts from Middlemount Coal Pty Ltd and other related parties			$ (2,300,000)	0
Equity interest percentage of revolving loans limit	50.00%		50.00%		50.00%
Revolving loan limit					$ 160,000,000
Financing receivable, stated interest rate (in percent)			10.00%
Intercompany loans, carrying value	$ 42,900,000		$ 42,900,000			$ 46,200,000
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
(Loss) income from equity method investments	$ (3,500,000)	$ (6,000,000.0)	$ (4,400,000)	$ (15,100,000)