Debt and Financing Arrangements (Details) $ / shares in Units, shares in Millions								3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 03, 2022 USD ($)	May 31, 2022 USD ($) shares	Jul. 29, 2021 USD ($) installment	Nov. 03, 2020 USD ($) $ / shares shares	Oct. 29, 2020 $ / shares	Jul. 02, 2020 USD ($)	Mar. 04, 2020 USD ($) installment	Sep. 30, 2022 USD ($) item $ / shares	Mar. 31, 2022 USD ($)	Sep. 30, 2022 USD ($) $ / shares	Sep. 30, 2021 USD ($)	Dec. 31, 2017 USD ($)	Aug. 02, 2022 USD ($)	Dec. 31, 2021 USD ($)	Mar. 04, 2021 USD ($)	Nov. 30, 2020 USD ($)
Debt Instrument [Line Items]
Term loan, face value																$ 155,300,000
Letters of credit outstanding								$ 95,400,000		$ 95,400,000
Interest rate																5.25%
Restricted cash								1,100,000		1,100,000	$ 1,101,000			$ 1,101,000
Debt financing costs										690,000	$ 2,057,000
Outstanding debt								174,093,000		174,093,000				560,673,000
Derivative Assets								3,540,000		3,540,000				6,104,000
Paid-in Capital
Debt Instrument [Line Items]
Cash on convertible notes																$ 39,200,000
Retained Earnings
Debt Instrument [Line Items]
Cash on convertible notes																$ 6,700,000
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
Term loan repaid										273,000,000.0
Amount of voting equity interests of domestic subsidiaries guaranteed (percent)												100
Amount of voting equity interests of foreign owned subsidiaries guaranteed (percent)												65
Outstanding debt								7,300,000		7,300,000
Loss on Early Repayment of Debt										4,100,000
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]
Gains on interest rate swap								600,000
Amount reclassified from other comprehensive income to expense									$ 1,400,000
Derivative Assets								1,400,000		1,400,000
Accounts Receivable Securitization Facility | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Current borrowing capacity								66,100,000		66,100,000
Letters of credit outstanding								69,200,000		69,200,000
Accounts Receivable Securitization Facility | Regions Bank | Line of Credit | Secured Debt
Debt Instrument [Line Items]
Current borrowing capacity	$ 150,000,000												$ 110,000,000
Inventory-Based Revolving Credit Facility
Debt Instrument [Line Items]
Covenant amount										23,800,000
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								26,200,000		26,200,000
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
Equipment financing arrangement | Secured Debt
Debt Instrument [Line Items]
Term loan, face value			$ 23,500,000				$ 53,600,000
Number of monthly payments | installment			42				48
Interest rate			7.35%				6.34%
Tax Exempt Bonds
Debt Instrument [Line Items]
Term loan, face value								$ 98,100,000		$ 98,100,000				$ 98,100,000
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				$ 34.151		$ 34.151
Stock price trigger (as a percent)								130.00%
Trading days, number | item								20
Convertible Notes exceeded the principal amount										$ 62,700,000
Consecutive trading days, period | item								30
Total interest expense								$ 300,000		4,300,000
Contractual interest coupon								200,000		3,800,000
Amortization of debt discount								100,000		500,000
Principal amount of convertible notes		$ 125,200,000						4,700,000
Cash on convertible notes		130,100,000						19,300,000		19,300,000
Accrued interest		$ 200,000
Deferred Offering Costs				$ 5,100,000
Unamortized deferred financing fees								3,500,000		3,500,000
Outstanding debt								25,400,000		25,400,000
Debt conversion, number of shares | shares		2.6
Loss on debt conversion										10,000,000.0
Inducement premium payment										5,000,000.0
Professional fees										1,500,000
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					$ 52.255
Capped call, transaction cost				$ 17,500,000
Debt instrument capped call transaction intrinsic value								$ 62,100,000		$ 62,100,000