Debt and Financing Arrangements (Narrative) (Details) - USD ($)						3 Months Ended	12 Months Ended
	Aug. 27, 2018	Apr. 27, 2018	Apr. 03, 2018	Apr. 27, 2017	Mar. 07, 2017	Mar. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Fair value of derivative assets						$ 62,857,000	$ 18,879,000
Interest Rate Swap
Line of Credit Facility [Line Items]
Interest rate swaps settlements gains						500,000
Other Noncurrent Assets | Interest Rate Swap
Line of Credit Facility [Line Items]
Fair value of derivative assets						900,000
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%
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	$ 294,000,000.0
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
Current borrowing capacity						$ 81,500,000
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						15,700,000
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
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						$ 14,300,000
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				2.00%
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%