Income Tax Matters - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred income tax assets:
Loss and credit carryforwards	$ 33.4	$ 48.7
Salaried VEBA (see Note 5)	4.8	8.7
Other assets	29.4	30.1
Leased assets	7.3	7.1
Inventories	6.9	9.4
Valuation allowances	(6.4)	(9.9)
Total deferred income tax assets	75.4	94.1
Deferred income tax liabilities:
Property, plant and equipment	(80.8)	(78.5)
Leased liability	(6.4)	(6.3)
Undistributed foreign earnings	(2.1)	(2.0)
Total deferred income tax liabilities	(89.3)	(86.8)
Net deferred income tax liabilities	$ (13.9)
Net deferred income tax assets		$ 7.3