Other Events Narrative (Details) $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 25, 2023 USD ($)	Oct. 02, 2023 USD ($)	Oct. 27, 2023 USD ($) shares	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($)	Sep. 30, 2022 AUD ($)
Other Commercial Events [Line Items]
Revenues				$ 1,078,900,000	$ 1,342,500,000	$ 3,711,700,000	$ 3,355,800,000
Other Noncash Income						9,400,000	0
Revenue from Collaborative Arrangement, Excluding Revenue from Contract with Customer								$ 30.0
Revenue related to excess port and rail capacity agreement						19,200,000
Common stock repurchase amount including commission fees				82,400,000		266,600,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 733,600,000		$ 733,600,000
Stock Repurchased During Period, Shares | shares				3,700,000		12,600,000
Subsequent Event
Other Commercial Events [Line Items]
Common stock repurchase amount including commission fees			$ 27,300,000
Stock Repurchased During Period, Shares | shares			1,100,000
Unsettled common stock repurchases		$ 2,600,000
Subsequent Event | Wards Well coal deposit
Other Commercial Events [Line Items]
Asset Acquisition, Price of Acquisition, Expected	$ 136,000,000
Contingent royalty	$ 200,000,000
Treasury Stock, Common
Other Commercial Events [Line Items]
Sales and Excise Tax Payable				$ 900,000	0	$ 2,500,000	0
Unsettled common stock repurchases				(8,600,000)	$ 0	2,600,000	$ 0
North Antelope Rochelle Mine
Other Commercial Events [Line Items]
Storm Damage Provision				$ 3,300,000		8,300,000
North Antelope Rochelle Mine | Materials and supplies inventories
Other Commercial Events [Line Items]
Storm Damage Provision						4,000,000
North Antelope Rochelle Mine | Buildings and equipment
Other Commercial Events [Line Items]
Storm damage provision for buildings and equipment						1,000,000
North Antelope Rochelle Mine | Incremental repair costs
Other Commercial Events [Line Items]
Storm Damage Provision						3,300,000
Shoal Creek Mine
Other Commercial Events [Line Items]
Business interruption and property loss limits on insurance claim, amount						125,000,000
Provision for loss related to fire						28,700,000
Deductible amount						50,000,000
Shoal Creek Mine | Longwall and development and other costs
Other Commercial Events [Line Items]
Provision for loss related to fire						17,800,000
Shoal Creek Mine | Equipment deemed inoperable
Other Commercial Events [Line Items]
Provision for loss related to fire						$ 10,900,000