Income Tax Matters, Components of Deferred Tax Assets and Liabilities Table (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets:
Loss and credit carryforwards	$ 191.8	$ 255.7
VEBAs (see Note 6)	23.2	25.9
Other assets	39.8	38.7
Valuation allowances	(15.7)	(21.2)
Total deferred income tax assets	239.1	299.1
Deferred income tax liabilities:
Property, plant and equipment	(82.7)	(79.6)
Inventories	0.0	(9.4)
Total deferred income tax liabilities	(82.7)	(89.0)
Net deferred income tax assets	156.4	210.1
Deferred Tax Assets, Net of Valuation Allowance, Current	0.0	49.6
Net deferred income tax assets, long term	159.7	162.6
Deferred tax liability, long term	$ (3.3)	(2.1)
Net deferred income tax assets, current		$ 49.6