Debt and Financing Arrangements (Company Debt Narrative) (Details) - USD ($)				3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 25, 2017	Apr. 27, 2017	Mar. 07, 2017	Sep. 30, 2017	Dec. 31, 2016	Oct. 01, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt			$ 0		$ 0		$ (2,547,000)
Letters of credit outstanding							7,400,000
Additional availability for borrowings							1,100,000
Derivative Assets					74,155,000		55,550,000	$ 74,155,000
Debt financing costs					0		10,149,000
Senior notes | New term loan debt facility
Debt Instrument [Line Items]
Amount of debt instrument	$ 300,000,000
Call premium	1.00%
Mandatory prepayment amount			100.00%
Default to indebtedness amount			$ 50,000,000
Default to surety, reclamation, or similar bonds securing obligations			50,000,000
Default to uninsured judgements			50,000,000
Uninsured losses or proceedings against minimum amount			50,000,000
Senior notes | New term loan debt facility | LIBOR
Debt Instrument [Line Items]
Applicable margin on interest rate	3.25%
Successor
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt					0		(2,547,000)
Debt financing costs					$ 0		10,100,000
Legal and advisory fees							2,500,000
Predecessor
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt						$ (2,213,000)			$ (27,910,000)
Debt financing costs						23,011,000			0
Legal and advisory fees						$ 2,200,000			$ 24,200,000
Predecessor | Line of credit
Debt Instrument [Line Items]
Write off of deferred financing costs								$ 3,700,000
Line of credit | Secured debt
Debt Instrument [Line Items]
Letters of credit outstanding							85,000,000
New term loan debt facility | Senior notes
Debt Instrument [Line Items]
Amount of debt instrument			$ 300,000,000
Percent of face amount of debt issued			99.50%
Quarterly principal amortization payments			$ 750,000
Voting interests of directly owned subsidiaries			100.00%
Voting equity interests of directly owned foreign subsidiaries			65.00%
New term loan debt facility | Senior notes | LIBOR
Debt Instrument [Line Items]
Applicable margin on interest rate			4.00%	3.25%
New term loan debt facility | Senior notes | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate	1.00%		1.00%
New term loan debt facility | Senior notes | Base rate
Debt Instrument [Line Items]
Applicable margin on interest rate	2.25%		3.00%
Regions Bank | Line of credit | Secured debt
Debt Instrument [Line Items]
Amount of debt instrument		$ 40,000,000
Maximum borrowing capacity							200,000,000
Current borrowing capacity		160,000,000
Liquidity amount minimum		$ 175,000,000
Letters of credit outstanding							29,000,000
Borrowing base percentage, coal inventory		85.00%
Borrowing base percentage, parts and supplies inventory		85.00%
Borrowing base, percentage of clause		35.00%
Percent of eligible cash		100.00%
Covenant amount		$ 250,000,000
Regions Bank | Line of credit | Secured debt | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate		2.25%
Regions Bank | Line of credit | Secured debt | LIBOR | Maximum
Debt Instrument [Line Items]
Borrowing base, percentage of clause		2.50%
Regions Bank | Line of credit | Secured debt | Base rate | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate		1.25%
Regions Bank | Line of credit | Secured debt | Base rate | Maximum
Debt Instrument [Line Items]
Applicable margin on interest rate		1.50%
Interest rate swap
Debt Instrument [Line Items]
Gain (loss) on interest rate swap							(100,000)
Interest rate swap | Other noncurrent assets
Debt Instrument [Line Items]
Derivative Assets							$ 1,800,000