Debt and Financing Arrangements (Narrative) (Details) - USD ($)						3 Months Ended	9 Months Ended	12 Months Ended
	Aug. 27, 2018	Apr. 27, 2018	Apr. 03, 2018	Apr. 27, 2017	Mar. 07, 2017	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Fair value of derivative assets						$ 42,646,000	$ 42,646,000	$ 18,879,000
Interest Rate Swap
Line of Credit Facility [Line Items]
Interest rate swaps settlements gains						200,000	1,200,000
Other Noncurrent Assets | Interest Rate Swap
Line of Credit Facility [Line Items]
Fair value of derivative assets						(3,400,000)	(3,400,000)
Senior Notes | New Term Loan Debt Facility
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
Senior Notes | New Term Loan Debt Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate			2.75%
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	$ 292,500,000.0	292,500,000.0
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%
Line of Credit | Secured Debt | Regions Bank
Line of Credit Facility [Line Items]
Maximum borrowing capacity								50,000,000
Facility size, increase								$ 10,000,000
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
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						$ 15,500,000	15,500,000
Current borrowing capacity						92,600,000	92,600,000
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						35,900,000	35,900,000
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						$ 14,100,000	$ 14,100,000
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				2.00%
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				1.00%
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%