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Financial Instruments and Other Guarantees - Textuals (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2019
Guarantee Obligations [Line Items]
Surety Bonds outstanding	$ 1,589.4		$ 1,589.4
Letters of credit outstanding, amount	283.6		283.6
Asset retirement obligations	665.8		665.8			$ 654.1
Line of Credit Facility, Remaining Borrowing Capacity	77.6		77.6			543.6
Net interest expense	34.3	$ 36.0	67.4	$ 71.8
Provision for North Goonyella equipment loss	0.0	0.0	0.0	24.7	$ 66.4
Obligations [Member]
Guarantee Obligations [Line Items]
Restricted Cash	0.0		0.0			0.0
Accounts Receivable Securitization Program, April 1, 2022 [Member]
Guarantee Obligations [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	10.5		10.5
Secured Debt \| Accounts Receivable Securitization Program, April 1, 2022 [Member]
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount	0.0		0.0
Letters of credit issued, amount	84.2		84.2
Line of Credit Facility, Maximum Borrowing Capacity	250.0		250.0
Cash collateral required	0.0		0.0			$ 0.0
Net interest expense	0.8	$ 1.0	1.5	$ 2.6
U.S Mining
Guarantee Obligations [Line Items]
Surety Bonds outstanding	1,398.9		1,398.9
Letters of credit outstanding, amount	185.4		185.4
Asset retirement obligations	$ 757.5		$ 757.5