Income Taxes Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Tax loss carryforwards and credits	$ 1,530.9	$ 1,729.3
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	276.6	304.5
Accrued postretirement benefit obligations	142.6	139.5
Asset retirement obligations	86.6	47.2
Employee benefits	25.3	24.8
Take or pay obligations	12.0	17.1
Investments and other assets	89.0	82.7
Workers’ compensation obligations	7.6	6.2
Operating lease right-of-use assets	20.8	0.0
Other	16.7	38.2
Total gross deferred tax assets	2,208.1	2,389.5
Valuation allowance, income tax	(2,068.4)	(2,094.3)
Total deferred tax assets	139.7	295.2
Deferred tax liabilities:
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	100.9	203.4
Operating lease liabilities	20.8	0.0
Coal supply agreements	3.1	9.3
Investments and other assets	15.4	43.7
Deferred Tax Liabilities, Net	(0.5)
Total deferred tax liabilities	140.2	256.4
Net deferred tax (liability) asset		38.8
Deferred taxes are classified as follows:
Noncurrent deferred income tax asset	28.3	48.5
Noncurrent deferred income tax liability	$ (28.8)	(9.7)
Net deferred tax (liability) asset		$ 38.8