Income Taxes - Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Tax loss carryforwards and credits	$ 740.1	$ 1,267.6
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	550.8	571.9
Accrued postretirement benefit obligations	41.4	48.9
Asset retirement obligations	95.1	91.4
Employee benefits	22.1	19.9
Take-or-pay obligations	8.2	9.5
Hedge activities	49.1	36.8
Interest limitation	0.0	7.9
Investments and other assets	37.0	81.8
Workers’ compensation obligations	7.1	7.2
Operating lease liabilities	7.8	11.3
Other	28.3	22.7
Total gross deferred tax assets	1,587.0	2,176.9
Valuation allowance, income tax	(1,451.0)	(2,120.8)
Total deferred tax assets	136.0	56.1
Deferred tax liabilities:
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	67.5	66.4
Operating lease right-of-use assets	7.6	9.4
Investments and other assets	6.6	7.6
Total deferred tax liabilities	81.7	83.4
Deferred Tax Assets, Net	54.3
Net deferred tax asset (liability)		(27.3)
Deferred taxes are classified as follows:
Noncurrent deferred income tax asset	74.7	0.0
Noncurrent deferred income tax liability	$ (20.4)	$ (27.3)