Equity Method Investments - Textual (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Equity Method Investments [Line Items]
Net income	$ 815.6	$ 1,319.1	$ 371.4
Changes in tax reserves	(0.8)	(1.5)	1.9
Revenue from physical sale of coal	4,946.7	4,981.9	3,318.3
Equity method investment, current assets	2,019.7	2,372.5
Equity method investment, current liabilities	979.0	918.7
Revolving Loans	0.0	0.0
Equity method investment related to Middlemount
Schedule of Equity Method Investments [Line Items]
Revenue from physical sale of coal	219.0	441.0	265.0
Equity method investment, current assets	48.2	75.2
Equity method investment ,noncurrent assets	199.5	256.3
Equity method investment, current liabilities	76.6	135.7
Equity method investment, noncurrent liabilities	58.2	97.7
Equity method investment related to Middlemount
Schedule of Equity Method Investments [Line Items]
Insurance recovery			12.5
Equity method investment related to Middlemount
Schedule of Equity Method Investments [Line Items]
Cash receipts from Middlemount	$ 0.0	168.4	$ 43.5
Equity interest percentage of revolving loans limit	50.00%
Financing receivable, stated interest rate (in percent)	10.00%
Equity method investment related to Middlemount
Schedule of Equity Method Investments [Line Items]
Valuation allowance	$ 33.0
Ownership percentage of equity method investment	50.00%
R3 Renewables
Schedule of Equity Method Investments [Line Items]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	$ 8.0	$ 10.9