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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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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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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Asset Impairment and Mine Closure Costs (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended	12 Months Ended
	Apr. 01, 2017	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Predecessor
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets Held-for-use								$ 247.9	$ 1,001.3
Equity Method Investment, Other than Temporary Impairment									276.5
Other than Temporary Impairment Losses, Investments	$ 0.0								276.5
Asset impairment	30.5	$ 30.5	$ 230.7	$ 17.2				247.9	1,277.8
Predecessor \| Contract Termination [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset Impairment Charges	$ 30.5
Predecessor \| Australian Mining
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets Held-for-use					$ 144.5	$ 230.5		193.2	675.2
Equity Method Investment, Other than Temporary Impairment									0.0
Asset impairment									675.2
Predecessor \| Australian Thermal Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets Held-for-use									17.5
Equity Method Investment, Other than Temporary Impairment									0.0
Asset impairment									17.5
Predecessor \| Midwestern U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets Held-for-use					9.7			54.7	40.2
Equity Method Investment, Other than Temporary Impairment									0.0
Asset impairment									40.2
Predecessor \| Corporate and Other
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets Held-for-use					$ 86.2			$ 37.5	268.4
Asset impairment									$ 544.9
Successor
Impaired Long-Lived Assets Held and Used [Line Items]
Other than Temporary Impairment Losses, Investments							$ 0.0
Asset impairment							0.0
Successor \| Contract Termination [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset Impairment Charges							$ 0.0