Debt and Financing Arrangements (Narrative) (Details)				3 Months Ended	12 Months Ended
	Mar. 04, 2020 USD ($) installment	Apr. 27, 2018	Mar. 07, 2017 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Apr. 27, 2017 USD ($)
Line of Credit Facility [Line Items]
Additional availability for borrowings				$ 17,500,000
Fair value of derivative assets				31,364,000			$ 31,616,000
Interest Rate Swap
Line of Credit Facility [Line Items]
Fair value of derivative assets				(5,600,000)
Interest rate swaps settlements gains				$ 200,000
Equipment financing arrangement
Line of Credit Facility [Line Items]
Interest rate	6.34%
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				$ 15,000,000.0
Line of Credit | Secured Debt | Regions Bank
Line of Credit Facility [Line Items]
Debt instrument, face amount						$ 40,000,000
Maximum borrowing capacity					$ 50,000,000
Covenant minimum amount						$ 175,000,000		$ 175,000,000
Letters of credit outstanding				32,400,000
Current borrowing capacity					160,000,000
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%
Facility size, increase					$ 10,000,000
Covenant amount (less than)						$ 250,000,000
Line of Credit | Secured Debt | LIBOR | Regions Bank | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate						2.00%
Line of Credit | Secured Debt | LIBOR | Regions Bank | Maximum
Line of Credit Facility [Line Items]
Percentage of clause borrowing base						2.50
Line of Credit | Secured Debt | Base Rate | Regions Bank | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate						1.00%
Line of Credit | Secured Debt | Base Rate | Regions Bank | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate						1.50%
Line of Credit | Secured Debt | Accounts Receivable Securitization Facility | Arch Receivable Company, LLC
Line of Credit Facility [Line Items]
Liquidity threshold, consecutive days		60 days
Liquidity threshold		364 days
Liquidity threshold, expiration period		60 days
Current borrowing capacity				$ 69,300,000