Income Taxes - Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Tax loss carryforwards and credits	$ 1,377.4	$ 1,530.9
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	573.7	276.6
Accrued postretirement benefit obligations	93.8	142.6
Asset retirement obligations	95.5	86.6
Employee benefits	22.8	25.3
Take-or-pay obligations	11.0	12.0
Investments and other assets	88.0	89.0
Workers’ compensation obligations	7.8	7.6
Operating lease liabilities	17.5	20.8
Other	24.1	16.7
Total gross deferred tax assets	2,311.6	2,208.1
Valuation allowance, income tax	(2,287.3)	(2,068.4)
Total deferred tax assets	24.3	139.7
Deferred tax liabilities:
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	36.2	100.9
Operating lease right-of-use assets	13.5	20.8
Coal supply agreements	3.1	3.1
Investments and other assets	1.6	15.4
Total deferred tax liabilities	54.4	140.2
Deferred Tax Liabilities, Net	(30.1)	(0.5)
Deferred taxes are classified as follows:
Noncurrent deferred income tax asset	4.9	28.3
Noncurrent deferred income tax liability	$ (35.0)	$ (28.8)