Debt and Financing Arrangements (Company Debt Narrative) (Details) $ / shares in Units, shares in Millions						1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Nov. 03, 2020 USD ($) $ / shares	Oct. 29, 2020 USD ($) $ / shares shares	Jul. 02, 2020 USD ($)	Mar. 04, 2020 USD ($) installment	Mar. 07, 2017 USD ($)	Aug. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2020 USD ($)
Debt Instrument [Line Items]
Letters of credit outstanding							$ 20,000,000.0		$ 20,000,000.0
Restricted cash							5,953,000		5,953,000	$ 0	$ 215,000
Net proceeds of convertible debt									155,250,000	0
Debt liability							508,312,000		508,312,000	310,819,000
Remaining proceeds									53,090,000	0
Derivative liabilities							2,399,000		2,399,000	25,870,000
Debt financing costs									9,718,000	0	$ 1,257,000
Equity components liability of the convertible notes									114,500,000
Equity components of the convertible notes							$ 40,800,000		$ 40,800,000
Hypothetical interest rate (as percent)							12.43%		12.43%
Discount amortized							$ 40,800,000		$ 40,800,000
Incurred offering-related costs							5,900,000		5,900,000
Debt issuance costs							1,900,000		$ 1,900,000	3,000,000.0
Amortized as interest expense over the term									5 years
Transaction costs									$ 1,500,000
New Term Loan Debt Facility | Senior Notes
Debt Instrument [Line Items]
Term loan, face value					$ 300,000,000
Percentage of face amount					99.50
Quarterly principal 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
Debt Instrument [Line Items]
Applicable margin on interest rate									2.75%
New Term Loan Debt Facility | Line of Credit
Debt Instrument [Line Items]
Call premium											1.00%
New Term Loan Debt Facility | Line of Credit | LIBOR
Debt Instrument [Line Items]
Applicable margin on interest rate											2.75%
New Term Loan Debt Facility | Line of Credit | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate											1.00%
New Term Loan Debt Facility | Line of Credit | Base Rate
Debt Instrument [Line Items]
Applicable margin on interest rate											1.75%
Interest Rate Swap
Debt Instrument [Line Items]
Derivative liabilities							3,900,000		$ 3,900,000
Gain (loss) on interest rate swap									(2,100,000)	$ 1,100,000	$ 1,200,000
New Term Loan Debt Facility | Senior Notes
Debt Instrument [Line Items]
Mandatory prepayment amount (as percent)					100
Default to indebtedness					$ 50,000,000
Default to surety, reclamation, or similar bonds securing obligations					50,000,000
Default to uninsured judgment					50,000,000
Uninsured losses or proceedings against minimum amount					$ 50,000,000
Accounts Receivable Securitization Facility | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Letters of credit outstanding							56,300,000		56,300,000
Accounts Receivable Securitization Facility | Regions Bank | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Current borrowing capacity								$ 110,000,000					$ 160,000,000
Accounts Receivable Securitization Facility | Arch Receivable Company, LLC | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Current borrowing capacity							25,400,000		25,400,000
Inventory-Based Revolving Credit Facility
Debt Instrument [Line Items]
Covenant amount									4,100,000
Inventory-Based Revolving Credit Facility | Regions Bank | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Term loan, face value												$ 50,000,000
Letters of credit outstanding							29,200,000		29,200,000
Borrowing base percentage, coal inventory												85
Borrowing base percentage, parts and supplies inventory												85
Borrowing base, percentage of clause												35
Percent of eligible cash												100.00%
Covenant amount						$ 175,000,000		$ 100,000,000				$ 100,000,000
Inventory-Based Revolving Credit Facility | Regions Bank | Line of Credit | Secured Debt | LIBOR | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate								2.50%
Inventory-Based Revolving Credit Facility | Regions Bank | Line of Credit | Secured Debt | LIBOR | Maximum
Debt Instrument [Line Items]
Applicable margin on interest rate								3.50%
Inventory-Based Revolving Credit Facility | Regions Bank | Line of Credit | Secured Debt | Base Rate | Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate								1.00%
Inventory-Based Revolving Credit Facility | Regions Bank | Line of Credit | Secured Debt | Base Rate | Maximum
Debt Instrument [Line Items]
Applicable margin on interest rate								1.50%
Equipment financing arrangement | Secured Debt
Debt Instrument [Line Items]
Term loan, face value				$ 53,600,000
Number of monthly payments | installment				48
Interest rate				6.34%
Tax Exempt Bonds
Debt Instrument [Line Items]
Term loan, face value			$ 53,100,000				53,100,000		53,100,000
Interest rate			5.00%
Mandatory tender percentage			100.00%
Proceeds from issuance of debt									47,100,000
Restricted cash							6,000,000.0		6,000,000.0
Tax Exempt Bonds | Minimum
Debt Instrument [Line Items]
Default to indebtedness								$ 50,000,000
Convertible Notes
Debt Instrument [Line Items]
Term loan, face value	$ 155,300,000
Interest rate	5.25%
Net proceeds of convertible debt	$ 132,700,000
Initial Conversion ratio	0.0267917
Initial Conversion price (in dollars per share) | $ / shares	$ 37.325
Stock price trigger (as a percent)	130.00%
Trading days, number	20
Consecutive trading days, period	30
Total interest expense							2,300,000
Contractual interest coupon							1,300,000
Amortization of debt discount							1,000,000.0
Incurred offering-related costs	$ 5,100,000
Debt issuance costs							$ 4,400,000		$ 4,400,000
Capped Call Transactions
Debt Instrument [Line Items]
Initial strike price (in dollars per share) | $ / shares		$ 37.325
Number of underlying shares | shares		4.2
Cap price | $ / shares		$ 52.2550
Premium percentage (as percent)		75.00%
Capped call, transaction cost	$ 17,500,000	$ 17,500,000