Equity Method Investments - Textual (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 AUD ($)
Schedule of Equity Method Investments [Line Items]
Cash receipts from Middlemount	$ (23,200,000)	$ (27,300,000)	$ (13,800,000)
Net (loss) income	(1,873,800,000)	(185,100,000)	663,800,000
Changes in tax reserves	(7,700,000)	3,000,000.0	2,100,000
Revenues	2,881,100,000	4,623,400,000	5,581,800,000
Equity method investment, current assets	1,420,300,000	1,613,900,000
Equity method investment, current liabilities	$ 790,600,000	975,300,000
Performance Units
Schedule of Equity Method Investments [Line Items]
Stock price measurement period	3 years
Return on invested capital period	3 years
Middlemount
Schedule of Equity Method Investments [Line Items]
Cash receipts from Middlemount	$ 0	14,700,000	106,700,000
Equity interest percentage of revolving loans limit	50.00%			50.00%
Revolving loan limit				$ 160
Financing receivable, stated interest rate (in percent)	10.00%
Intercompany loans, carrying value	$ 46,200,000	17,500,000
Middlemount
Schedule of Equity Method Investments [Line Items]
Net (loss) income			9,000,000
Changes in tax reserves			17,000,000
Valuation allowance	33,000,000
Revenues	123,000,000	160,000,000	$ 271,000,000
Equity method investment, current assets	31,000,000.0	30,800,000
Equity method investment ,noncurrent assets	$ 301,800,000	209,700,000
Ownership percentage of equity method investment	50.00%			50.00%
Equity method investment, current liabilities	$ 273,800,000	225,800,000
Equity method investment, noncurrent liabilities	$ 83,500,000	$ 40,100,000