Debt and Financing Arrangements (Narrative) (Details) - USD ($)				3 Months Ended
	Sep. 25, 2017	Apr. 27, 2017	Mar. 07, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Line of Credit Facility [Line Items]
Net loss resulting from early retirement of debt and debt restructuring				$ 0	$ (2,030,000)
Fair value of derivative assets				32,762,000		$ 55,550,000
Debt financing costs				0	7,228,000
Debt restructuring costs				0	2,030,000
Interest Rate Swap
Line of Credit Facility [Line Items]
Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net				100,000
Interest Rate Swap | Other Noncurrent Assets
Line of Credit Facility [Line Items]
Fair value of derivative assets				$ 3,700,000
New Term Loan Debt Facility | Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 300,000,000	$ 300,000,000
Basis spread on variable rate (percent)	3.25%
Net cash proceeds of from debt issuances and other non-ordinary sales or dispositions (percent)			100.00%
Default to indebtedness			$ 50,000,000
Default to surety, reclamation or similar bond			50,000,000
Default to uninsured judgment			50,000,000
Uninsured losses or proceedings against			50,000,000
Call premium rate	1.00%
New Term Loan Debt Facility | Senior Notes | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.00%
Predecessor | Superpriority Debtor-in Possession Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Debt financing costs					$ 7,200,000
New Term Loan Debt Facility | Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, face amount			$ 300,000,000
Percentage of face amount			99.50%
Quarterly amortization payments			$ 750,000
Amount of voting equity interests of domestic subsidiaries guaranteed (percent)			100.00%
Amount of voting equity interests of foreign owned subsidiaries guaranteed (percent)			65.00%
New Term Loan Debt Facility | Senior Notes | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)				3.25%
New Term Loan Debt Facility | Senior Notes | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	2.25%
Line of Credit | Secured Debt
Line of Credit Facility [Line Items]
Letters of credit outstanding				$ 0
Current borrowing capacity				0
Cash collateral for borrowed securities				0
Line of Credit | Regions Bank | Secured Debt
Line of Credit Facility [Line Items]
Debt instrument, face amount		$ 40,000,000
Percentage of coal inventory borrowing base		85.00%
Percentage of parts and supplies inventory borrowing base		85.00%
Percentage of clause borrowing base		35.00%
Percent of eligible cash		100.00%
Covenant amount (less than)		$ 250,000,000
Covenant minimum amount		175,000,000
Letters of credit outstanding				35,200,000
Additional availability for borrowings				4,800,000
Maximum borrowing capacity		$ 200,000,000
Current borrowing capacity				$ 160,000,000
Line of Credit | Regions Bank | Minimum | Secured Debt | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)		2.25%
Line of Credit | Regions Bank | Minimum | Secured Debt | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)		1.25%
Line of Credit | Regions Bank | Maximum | Secured Debt | LIBOR
Line of Credit Facility [Line Items]
Percentage of clause borrowing base		2.50%
Line of Credit | Regions Bank | Maximum | Secured Debt | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)		1.50%