Segment Information - Reconciliation to Adjusted EBITDA (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Loss from continuing operations, net of income taxes	$ (77.7)	$ (129.3)
Depreciation, depletion and amortization	68.3	106.0
Asset retirement obligation expenses	15.9	17.6
Restructuring charges	2.1	6.5
Transaction costs related to joint ventures	0.0	4.2
Changes in deferred tax asset valuation allowance and reserves and amortization of basis difference related to equity affiliates	(1.5)	(0.7)
Interest expense	52.4	33.1
Gain on early debt extinguishment	(3.5)	0.0
Interest income	(1.5)	(3.1)
Unrealized losses on economic hedges	1.9	2.2
Unrealized losses (gains) on non-coal trading derivative contracts	7.6	(0.1)
Take-or-pay contract-based intangible recognition	(1.1)	(2.6)
Income tax (benefit) provision	(1.8)	3.0
Adjusted EBITDA	$ 61.1	$ 36.8