Segment Information Reconciliation to Adjusted EBITDA (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
Segment Reporting [Abstract]
Income from continuing operations, net of income taxes	$ 42.9	$ 120.0	$ 176.2	$ 328.3
Depreciation, depletion and amortization	165.4	163.9	337.9	333.5
Asset retirement obligation expenses	15.3	13.2	29.1	25.5
Provision for North Goonyella equipment loss	0.0	0.0	24.7	0.0	$ 66.4
North Goonyella insurance recovery - equipment (1)	0.0	0.0	(91.1)	0.0
Changes in deferred tax asset valuation allowance and reserves and amortization of basis difference related to equity affiliates	0.3	(8.4)	0.3	(16.0)
Interest expense	36.0	38.3	71.8	74.6
Loss on early debt extinguishment	0.0	2.0	0.0	2.0
Interest income	(7.2)	(7.0)	(15.5)	(14.2)
Reorganization items, net	0.0	0.0	0.0	(12.8)
Unrealized (gains) losses on economic hedges	(22.4)	48.1	(62.2)	9.5
Unrealized losses (gains) on non-coal trading derivative contracts	0.3	(0.1)	0.1	1.7
Fresh start take-or-pay contract-based intangible recognition	(5.6)	(7.8)	(11.2)	(16.1)
Income tax provision	3.0	7.4	21.8	17.5
Total Adjusted EBITDA	228.0	369.6	481.9	733.5
Insurance recovery related to North Goonyella equipment losses	$ 0.0	$ 0.0	125.0	$ 0.0
Business Interruption Losses			$ 33.9