Income Tax Matters, Components of Deferred Tax Assets and Liabilities Table (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred income tax assets:
Loss and credit carryforwards	$ 255.7	$ 275.4
VEBAs (see Note 6)	25.9	5.1
Other assets	38.7	37.8
Inventories	0.0	18.7
Valuation allowances	(21.2)	(19.2)
Total deferred income tax assets	299.1	317.8
Deferred income tax liabilities:
Property, plant and equipment	(79.6)	(74.1)
VEBAs (see Note 6)	0.0	(120.6)
Inventories	(9.4)	(6.7)
Total deferred income tax liabilities	(89.0)	(201.4)
Net deferred income tax assets	210.1	116.4
Deferred Tax Assets, Net of Valuation Allowance, Current	49.6	86.4
Net deferred income tax assets, long term	162.6	30.9
Deferred tax liability, long term	$ (2.1)	(0.9)
Net deferred income tax assets, current		$ 86.4