Income Taxes - Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Tax loss carryforwards and credits	$ 1,267.6	$ 1,377.4
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	571.9	573.7
Accrued postretirement benefit obligations	48.9	93.8
Asset retirement obligations	91.4	95.5
Employee benefits	19.9	22.8
Take-or-pay obligations	9.5	11.0
Hedge activities	36.8	2.4
Interest limitation	7.9	0.0
Investments and other assets	81.8	88.0
Workers’ compensation obligations	7.2	7.8
Operating lease liabilities	11.3	17.5
Other	22.7	21.7
Total gross deferred tax assets	2,176.9	2,311.6
Valuation allowance, income tax	(2,120.8)	(2,287.3)
Total deferred tax assets	56.1	24.3
Deferred tax liabilities:
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	66.4	36.2
Operating lease right-of-use assets	9.4	13.5
Investments and other assets	7.6	4.7
Total deferred tax liabilities	83.4	54.4
Deferred Tax Liabilities, Net	(27.3)	(30.1)
Deferred taxes are classified as follows:
Noncurrent deferred income tax asset	0.0	4.9
Noncurrent deferred income tax liability	$ (27.3)	$ (35.0)