Debt and Financing Arrangements (Narrative) (Details)				3 Months Ended	9 Months Ended	12 Months Ended
	Jul. 02, 2020 USD ($)	Mar. 04, 2020 USD ($) installment	Mar. 07, 2017 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2018	Dec. 31, 2017 USD ($)	Aug. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Line of Credit Facility [Line Items]
Restricted cash				$ 13,919,000	$ 13,919,000
Additional availability for borrowings					14,800,000
Fair value of derivative assets				10,422,000	10,422,000					$ 31,616,000
Interest Rate Swap
Line of Credit Facility [Line Items]
Fair value of derivative assets				(4,500,000)	(4,500,000)
Interest rate swaps settlements gains				600,000	1,500,000
Equipment financing arrangement
Line of Credit Facility [Line Items]
Interest rate		6.34%
Tax Exempt Bonds
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 53,100,000
Interest rate	5.00%
Mandatory tender percentage	100.00%
Proceeds from issuance of debt				38,100,000
Restricted cash				13,900,000	$ 13,900,000
Tax Exempt Bonds | Minimum
Line of Credit Facility [Line Items]
Covenant for cross-events of default to indebtedness				50,000,000
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
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%
Senior Notes | New Term Loan Debt Facility | LIBOR | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate						1.00%
Secured Debt | Equipment financing arrangement
Line of Credit Facility [Line Items]
Debt instrument, face amount		$ 53,600,000
Number of monthly payments | installment		48
Line of Credit | 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
Amount of voting equity interests of foreign owned subsidiaries guaranteed (percent)			65
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
Line of Credit Facility [Line Items]
Letters of credit outstanding				50,000,000.0	$ 50,000,000.0
Line of Credit | Secured Debt | Regions Bank
Line of Credit Facility [Line Items]
Covenant minimum amount							$ 100,000,000
Letters of credit outstanding				29,200,000	29,200,000
Current borrowing capacity				110,000,000	110,000,000				$ 160,000,000
Line of Credit | Secured Debt | Accounts Receivable Securitization Facility | Arch Receivable Company, LLC
Line of Credit Facility [Line Items]
Current borrowing capacity				34,500,000	$ 34,500,000
Line of Credit | Secured Debt | Inventory-Based Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Debt instrument, face amount							$ 50,000,000
Percentage of coal inventory borrowing base							85
Percentage of parts and supplies inventory borrowing base							85
Percentage of clause borrowing base							35
Percent of eligible cash							100.00%
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					2.50%
Line of Credit | Secured Debt | Inventory-Based Revolving Credit Facility | LIBOR | Regions Bank | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate					3.50%
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%
Line of Credit | Line of Credit | Inventory-Based Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Covenant minimum amount				$ 100,000,000	$ 100,000,000			$ 175,000,000