Leases - Textuals (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Provision for North Goonyella equipment loss	$ 24.7	$ 0.0		$ 66.4
Operating Leases, Rent Expense				$ 44.7
Leased Equipment
Lessee, Lease, Description [Line Items]
Provision for North Goonyella equipment loss			$ 50.7
Minimum
Lessee, Lease, Description [Line Items]
Operating and Finance Leases Remaining Lease Term	1 year
Maximum
Lessee, Lease, Description [Line Items]
Operating and Finance Leases Remaining Lease Term	23 years