Leases Textuals (Details) - USD ($) $ in Millions	3 Months Ended					9 Months Ended	12 Months Ended		15 Months Ended
	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Operating Leased Assets [Line Items]
Lease Obligation, Assumed Amount Recoverable from Third Parties	$ 0.0						$ 0.0		$ 0.0
Provision for equipment loss	58.5	$ 24.7	$ 17.1	$ 49.3	$ 0.0	$ 0.0	83.2	$ 66.4
Minimum Annual Production Of Federal Coal Mining Leases On Original Amount								1.00%
Capital lease and other obligations	$ 15.2		$ 40.0				$ 15.2	$ 40.0	15.2
North Goonyella Mine | Leased Equipment
Operating Leased Assets [Line Items]
Provision for equipment loss								$ 50.7	$ 50.7
Maximum
Operating Leased Assets [Line Items]
Operating and Finance Leases Remaining Lease Term							11 years 3 months 18 days
Minimum
Operating Leased Assets [Line Items]
Operating and Finance Leases Remaining Lease Term							1 year