Debt and Financing Arrangements (Company Debt Narrative) (Details) - USD ($)			12 Months Ended
	Apr. 27, 2018	Mar. 07, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 27, 2017
Debt Instrument [Line Items]
Letters of credit outstanding			$ 0
Additional availability for borrowings			14,100,000
Derivative liabilities			25,870,000	$ 31,089,000
Debt financing costs			0	$ 1,257,000	$ 10,149,000
Senior notes | Term loan debt facility
Debt Instrument [Line Items]
Mandatory prepayment amount		100.00%
Default to indebtedness amount		$ 50,000,000
Default to surety, reclamation, or similar bonds securing obligations		50,000,000
Default to uninsured judgements		50,000,000
Uninsured losses or proceedings against minimum amount		50,000,000
Line of credit | Secured debt
Debt Instrument [Line Items]
Letters of credit outstanding			$ 15,500,000
Term loan debt facility | Senior notes
Debt Instrument [Line Items]
Amount of debt instrument		$ 300,000,000
Percent of face amount of debt issued		99.50%
Quarterly principal amortization payments		$ 750,000
Voting interests of directly owned subsidiaries		100.00%
Voting equity interests of directly owned foreign subsidiaries		65.00%
Term loan debt facility | Senior notes | LIBOR
Debt Instrument [Line Items]
Applicable margin on interest rate			2.75%
Term loan debt facility | Line of credit
Debt Instrument [Line Items]
Call premium				1.00%
Term loan debt facility | Line of credit | LIBOR
Debt Instrument [Line Items]
Applicable margin on interest rate				2.75%
Term loan debt facility | Line of credit | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate				1.00%
Term loan debt facility | Line of credit | Base rate
Debt Instrument [Line Items]
Applicable margin on interest rate				1.75%
Regions Bank | Line of credit | Secured debt
Debt Instrument [Line Items]
Amount of debt instrument					40,000,000
Maximum borrowing capacity				$ 50,000,000
Current borrowing capacity				160,000,000
Liquidity amount minimum					$ 175,000,000	$ 175,000,000
Letters of credit outstanding			$ 35,900,000
Borrowing base percentage, coal inventory					85.00%
Borrowing base percentage, parts and supplies inventory					85.00%
Borrowing base, percentage of clause					35.00%
Percent of eligible cash					100.00%
Increase to line of credit facility				10,000,000
Covenant amount					$ 250,000,000
Regions Bank | Line of credit | Secured debt | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate					2.00%
Regions Bank | Line of credit | Secured debt | LIBOR | Maximum
Debt Instrument [Line Items]
Borrowing base, percentage of clause					2.50%
Regions Bank | Line of credit | Secured debt | Base rate | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate					1.00%
Regions Bank | Line of credit | Secured debt | Base rate | Maximum
Debt Instrument [Line Items]
Applicable margin on interest rate					1.50%
Arch Receivable Company, LLC | Line of credit | Secured debt | Accounts Receivable Securitization Facility
Debt Instrument [Line Items]
Current borrowing capacity			108,900,000
Maturity date	3 years
Liquidity threshold, consecutive days	60 days
Liquidity threshold	364 days
Liquidity threshold, expiration period	60 days
Interest rate swap
Debt Instrument [Line Items]
Derivative liabilities			3,000,000.0
Gain (loss) on interest rate swap			$ 1,100,000	$ 1,200,000	$ (100,000)