Debt and Financing Arrangements (Narrative) (Details) - USD ($)							3 Months Ended	9 Months Ended
	Aug. 27, 2018	Apr. 27, 2018	Apr. 03, 2018	Sep. 25, 2017	Apr. 27, 2017	Mar. 07, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Line of Credit Facility [Line Items]
Fair value of derivative assets							$ 50,526,000	$ 50,526,000		$ 55,550,000
Debt financing costs								1,009,000	$ 10,043,000
Debt restructuring costs								50,000	$ 2,360,000
Interest Rate Swap
Line of Credit Facility [Line Items]
Interest rate swaps settlements gains							300,000	800,000
Other Noncurrent Assets | Interest Rate Swap
Line of Credit Facility [Line Items]
Fair value of derivative assets							$ 3,600,000	3,600,000
Senior Notes | New Term Loan Debt Facility
Line of Credit Facility [Line Items]
Debt instrument, face amount				$ 300,000,000
Basis spread on variable rate				3.25%
Call premium rate				1.00%
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
Debt financing costs								500,000
Senior Notes | New Term Loan Debt Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate			2.75%	1.00%
Call premium rate			1.00%
Basis spread on variable rate, floor			1.00%
Senior Notes | New Term Loan Debt Facility | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate			1.75%
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							2.75%
New Term Loan Debt Facility | Senior Notes | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate				2.25%
Line of Credit | Secured Debt
Line of Credit Facility [Line Items]
Debt financing costs								500,000
Line of Credit | Secured Debt | Accounts Receivable Securitization Facility | Arch Receivable Company, LLC
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 160,000,000
Maturity date	3 years	3 years
Current borrowing capacity							$ 83,300,000	83,300,000
Covenant minimum amount					$ 175,000,000
Liquidity threshold, consecutive days		60 days
Liquidity threshold		364 days
Liquidity threshold, expiration period		60 days
Letters of credit outstanding							75,000,000	75,000,000
Line of Credit | Secured Debt | Inventory-Based Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Debt instrument, face amount					40,000,000
Covenant minimum amount					$ 175,000,000
Liquidity threshold, consecutive days		60 days
Liquidity threshold		364 days
Liquidity threshold, expiration period		60 days
Letters of credit outstanding							35,700,000	35,700,000
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
Additional availability for borrowings							$ 4,300,000	$ 4,300,000
Line of Credit | Secured Debt | Inventory-Based Revolving Credit Facility | LIBOR | Regions Bank | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate					2.25%
Line of Credit | Secured Debt | Inventory-Based Revolving Credit Facility | LIBOR | Regions Bank | Maximum
Line of Credit Facility [Line Items]
Percentage of clause borrowing base					2.50%
Line of Credit | Secured Debt | Inventory-Based Revolving Credit Facility | Base Rate | Regions Bank | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate					1.25%
Line of Credit | Secured Debt | Inventory-Based Revolving Credit Facility | Base Rate | Regions Bank | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate					1.50%