Debt and Financing Arrangements (Narrative) (Details) - USD ($)			3 Months Ended		6 Months Ended
	Apr. 27, 2017	Mar. 07, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Line of Credit Facility [Line Items]
Net loss resulting from early retirement of debt and debt restructuring		$ 0
Fair value of derivative assets			$ 42,661,000		$ 42,661,000		$ 74,155,000
Debt restructuring costs					2,100,000	$ 2,200,000
Interest Rate Swap | Other Noncurrent Assets
Line of Credit Facility [Line Items]
Fair value of derivative assets			200,000		200,000
New Term Loan Debt Facility | Senior Notes
Line of Credit Facility [Line Items]
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
Previous First Lien Debt Facility | Senior Notes
Line of Credit Facility [Line Items]
Debt restructuring costs					1,000,000
Successor
Line of Credit Facility [Line Items]
Net loss resulting from early retirement of debt and debt restructuring			(31,000)		(2,061,000)
Debt financing costs					8,900,000
Debt restructuring costs					2,061,000
Successor | New Term Loan Debt Facility | Senior Notes
Line of Credit Facility [Line Items]
Debt financing costs					7,200,000
Predecessor
Line of Credit Facility [Line Items]
Net loss resulting from early retirement of debt and debt restructuring				$ 0		(2,213,000)
Debt financing costs						18,806,000
Debt restructuring costs						2,213,000
Predecessor | Unsecured Debt
Line of Credit Facility [Line Items]
Interest expense, debt				$ 100,800,000		199,300,000
Predecessor | Superpriority Debtor-in Possession Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Debt financing costs						$ 18,800,000
Line of Credit | Secured Debt
Line of Credit Facility [Line Items]
Letters of credit outstanding	$ 130,400,000
Current borrowing capacity			88,700,000		88,700,000
Cash collateral for borrowed securities	41,700,000
Line of Credit | Successor | Secured Debt
Line of Credit Facility [Line Items]
Debt financing costs					1,700,000
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	$ 250,000,000
Covenant minimum amount	175,000,000
Letters of credit outstanding			21,200,000		21,200,000
Maximum borrowing capacity			$ 200,000,000		$ 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%
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)		4.00%	4.00%
New Term Loan Debt Facility | Senior Notes | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)		3.00%
New Term Loan Debt Facility | Minimum | Senior Notes | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)		1.00%