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Financial Instruments and Other Guarantees - Textuals (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Guarantee Obligations [Line Items]
Surety Bonds outstanding	$ 1,557.4
Letters of credit outstanding, amount	206.8
Asset retirement obligations	666.6			$ 654.1
Net interest expense	33.1	$ 35.8
Provision for North Goonyella equipment loss	0.0	24.7	$ 66.4
Accounts Receivable Securitization Program, April 1, 2022 [Member]
Guarantee Obligations [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	12.8
Secured Debt \| Accounts Receivable Securitization Program, April 1, 2022 [Member]
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount	0.0
Letters of credit issued, amount	132.7
Line of Credit Facility, Maximum Borrowing Capacity	250.0
Cash collateral required	0.0			$ 0.0
Net interest expense	0.7	$ 1.1
U.S Mining
Guarantee Obligations [Line Items]
Surety Bonds outstanding	1,365.0
Letters of credit outstanding, amount	111.1
Asset retirement obligations	$ 758.7