Financial Instruments and Other Guarantees - Textuals (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Guarantee Obligations [Line Items]
Surety Bonds outstanding	$ 1,572.0		$ 1,572.0
Letters of credit outstanding, amount	202.2		202.2
Asset retirement obligations	698.9		698.9		$ 686.4
Net interest expense	36.0	$ 38.3	71.8	$ 74.6
Net impact on collateral requirements			0.0	323.1
Provision for North Goonyella equipment loss	0.0	0.0	24.7	0.0	$ 66.4
Proceeds from Release of Collateral, Short-term and Long-term				363.2
Secured Debt | Accounts Receivable Securitization Program, April 3, 2020
Guarantee Obligations [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	250.0		250.0
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	129.9		129.9
Cash collateral required	0.0		0.0
Net interest expense	1.0	$ 1.9	2.6	$ 3.8
U.S Mining
Guarantee Obligations [Line Items]
Surety Bonds outstanding	1,359.9		1,359.9
Letters of credit outstanding, amount	110.5		110.5
Asset retirement obligations	$ 762.8		$ 762.8