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 ($)	Sep. 30, 2020 USD ($)	Aug. 31, 2020 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2018	Dec. 31, 2017 USD ($)	Mar. 04, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)
Debt Instrument [Line Items]
Restricted cash								$ 18,962,000						$ 5,953,000
Net proceeds of convertible debt								155,300,000
Derivative liabilities								2,889,000						$ 2,399,000
Debt financing costs								1,194,000	$ 422,000
Equity components liability of the convertible notes								114,500,000
Equity components of the convertible notes								$ 40,800,000
Hypothetical interest rate (as percent)								12.43%
Discount amortized								$ 40,800,000
Deferred Offering Costs								5,900,000
Debt issuance costs								$ 4,400,000
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 | 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								$ 3,200,000
Gain (loss) on interest rate swap								(600,000)	$ (200,000)
Accounts Receivable Securitization Facility | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Letters of credit outstanding								58,300,000
Cash collateral								2,600,000
Accounts Receivable Securitization Facility | Regions Bank | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Current borrowing capacity						$ 110,000,000				$ 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								55,700,000
Inventory-Based Revolving Credit Facility
Debt Instrument [Line Items]
Covenant amount								12,100,000
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
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						$ 100,000,000	$ 175,000,000					$ 100,000,000
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								98,100,000
Proceeds from issuance of debt								81,800,000
Restricted cash								16,300,000
Tax Exempt Bonds | Minimum
Debt Instrument [Line Items]
Covenant for cross-events of default to indebtedness										$ 50,000,000
Series 2020 Tax Exempt Bonds
Debt Instrument [Line Items]
Term loan, face value			$ 53,100,000
Interest rate			5.00%
Mandatory tender percentage			100.00%
Series 2021 Tax Exempt Bonds
Debt Instrument [Line Items]
Term loan, face value													$ 45,000,000.0
Interest rate													4.125%
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	0.267917
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								3,700,000
Contractual interest coupon								2,000,000.0
Amortization of debt discount								$ 1,700,000
Deferred Offering Costs	$ 5,100,000
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