Gain on formation of United Wambo JV

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Mark-to-Market Adjustment on Pension and Postretirement Plans, Net

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Provision for equipment loss

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[Line Items] for Quarterly Financial Information [Table]

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Settlement on Coal Supply Agreement Termination

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

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Amount of gain (loss) on sale or disposal of assets, including but not limited to property plant and equipment, intangible assets and equity in securities of subsidiaries or equity method investee.

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- Definition

The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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The amount recovered from insurance. These recoveries reduce costs and losses that are reported as a separate line item under operating expenses.

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Amount of the cost of borrowed funds accounted for as interest expense.

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Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

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Total amount of reorganization items.

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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

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Amount, after cash payment, of expenses associated with exit or disposal activities pursuant to an authorized plan and expenses resulting from the write-down of assets. Excludes expenses related to a business combination, a discontinued operation or an asset retirement obligation.

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Summary Quarterly Financial Information Textuals (Details) - USD ($) $ in Millions			1 Months Ended	3 Months Ended									9 Months Ended	12 Months Ended
	Nov. 28, 2017	Nov. 27, 2017	Jul. 31, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information [Line Items]
Insurance Recoveries							$ 125.0					$ 0.0	$ 0.0	$ 125.0	$ 0.0
Gain on formation of United Wambo JV				$ 48.1								0.0	0.0	48.1	0.0
Settlement on Coal Supply Agreement Termination				69.3	$ 14.0
Acquisition costs related to Arch Joint Venture				11.8	8.2			$ 4.9				0.0	0.0	21.6	7.4
Asset impairment				250.2	20.0							30.5	0.0	270.2	0.0
Provision for North Goonyella equipment loss				58.5			24.7	17.1	$ 49.3			0.0	0.0	83.2	66.4
Restructuring charges	$ 3.0			23.0								0.0	7.6	24.3	1.2
Income (Loss) from Equity Method Investments				10.9	20.7	$ 9.7			17.2	$ 25.2	$ 22.0	15.0	49.0	3.4	68.1
Interest expense				36.8	35.4	36.0	35.8	36.5	38.2	38.3	36.3	32.9	119.7	144.0	149.3
Investment Income, Interest				4.5	$ 7.0	$ 7.2	$ 8.3	9.3	10.1	$ 7.0	7.2	2.7	5.6	27.0	33.6
Net mark-to-market adjustment on actuarially determined liabilities				$ 67.4				$ 125.5				0.0	(45.2)	67.4	(125.5)
Gain (Loss) on Disposition of Assets									20.7		30.6	22.8	84.0	2.1	48.2
Restructuring Costs and Asset Impairment Charges												30.5	0.0	270.2	0.0
Reorganization items, net											12.8	627.2	0.0	0.0	(12.8)
Loss on early debt extinguishment												0.0	(20.9)	(0.2)	(2.0)
Burton Mine
Quarterly Financial Information [Line Items]
Gain (Loss) on Disposition of Assets		$ 52.2
Gain (Loss) on Sale of Properties												0.0	52.2	0.0	0.0
Springfield Reclamation Liability
Quarterly Financial Information [Line Items]
Gain (Loss) on Disposition of Assets													31.2
Gain (Loss) on Sale of Properties												$ 0.0	$ 31.2	$ 0.0	$ 0.0
Iffley Land Transaction
Quarterly Financial Information [Line Items]
Gain (Loss) on Disposition of Assets											$ 20.6
Stanmore Coal Limited
Quarterly Financial Information [Line Items]
Gain (Loss) on Disposition of Assets			$ 20.5						$ 20.5