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Investments Textuals (Details) $ in Millions, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Investments Textuals (Details) $ in Millions, $ in Millions	Apr. 01, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Aug. 31, 2020 AUD ($)	Feb. 12, 2020 AUD ($)	Dec. 31, 2019 AUD ($)
Schedule of Equity Method Investments [Line Items]
Changes in tax reserves	$ (9.2)	$ 7.2	$ 3.0	$ 2.1
Equity method investment	0.0	0.0	$ 9.0	0.0
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Ownership percentage of equity method investment			50.00%				50.00%
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Intercompany loans							$ 50.0
Financing Receivable, Stated Interest Rate			15.00%
Intercompany loans, carrying value			$ 17.1	0.0
Financing Receivable \| Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Financing Receivable, Cash received from Equity Interest	32.7	48.1	$ 14.7	106.7
Equity method investment and financing receivables related to Middlemount
Schedule of Equity Method Investments [Line Items]
Ownership percentage of equity method investment			50.00%				50.00%
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Net Income (Loss)				9.0
Changes in tax reserves				17.0
Equity Method Investment, Summarized Financial Information, Revenue	$ 60.0	$ 193.0	$ 160.0	271.0
Equity Method Investment, Summarized Financial Information, Current Assets			30.8	33.6
Equity Method Investment, Summarized Financial Information, Noncurrent Assets			209.7	181.8
Equity Method Investment, Summarized Financial Information, Current Liabilities			225.8	187.9
Equity Method Investment, Summarized Financial Information, Noncurrent Liabilities			$ 40.1	$ 35.8
Scenario, Forecast \| Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Intercompany loans					$ 50.0	$ 70.0