Segment Information Reconciliation to Adjusted EBITDA (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting [Abstract]
Income from continuing operations, net of income taxes	$ 133.3	$ 208.3
Depreciation, depletion and amortization	172.5	169.6
Asset retirement obligation expenses	13.8	12.3
Provision for North Goonyella equipment loss	24.7	0.0	$ 66.4
North Goonyella insurance recoveries - equipment (1)	(91.1)	0.0
Changes in deferred tax asset valuation allowance and reserves and amortization of basis difference related to equity affiliates	0.0	(7.6)
Interest expense	35.8	36.3
Interest income	(8.3)	(7.2)
Reorganization items, net	0.0	(12.8)
Unrealized gains on economic hedges	(39.8)	(38.6)
Unrealized (gains) losses on non-coal trading derivative contracts	(0.2)	1.8
Fresh start take-or-pay contract-based intangible recognition	(5.6)	(8.3)
Income tax provision	18.8	10.1
Total Adjusted EBITDA	253.9	363.9
Insurance Recoveries	125.0	$ 0.0
Business Interruption Losses	$ 33.9