Income Tax Matters, Components of Deferred Tax Assets and Liabilities Table (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Loss and credit carryforwards	$ 98.7	$ 191.8
VEBAs (see Note 4)	11.6	23.2
Other assets	21.0	39.8
Inventory	9.3	0.0
Valuation allowances	(13.0)	(15.7)
Total deferred income tax assets	127.6	239.1
Deferred income tax liabilities:
Property, plant and equipment	(57.7)	(82.7)
Undistributed foreign earnings	(2.2)	0.0
Total deferred income tax liabilities	(59.9)	(82.7)
Net deferred income tax assets	67.7	156.4
Net deferred income tax assets, long term	72.0	159.7
Deferred tax liability, long term	$ (4.3)	$ (3.3)