Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Cash and Cash Equivalents
Cash and money market funds	$ 37.1	$ 29.5
Commercial paper	45.5	148.2
Total	82.6	177.7
Trade Receivables – Net
Trade receivables, gross	132.4	130.1
Allowance for doubtful receivables	(0.8)	(0.8)
Trade receivables – net	131.6	129.3
Inventories
Finished products	63.1	73.6
Work-in-process	82.2	66.7
Raw materials	49.6	54.2
Operating supplies and repair and maintenance parts	22.4	20.2
Total	217.3	214.7
Prepaid Expenses and Other Current Assets
Current derivative assets – Notes 8 and 9	0.3	85.7
Current deferred tax assets	92.2	86.4
Short-term restricted cash	0.3	0.3
Prepaid taxes	1.1	0.0
Other	4.7	6.2
Total	98.6	178.6
Property, Plant and Equipment – Net
Land and improvements	22.7	22.9
Buildings and leasehold improvements	68.4	63.8
Machinery and equipment	528.1	509.8
Construction in progress	42.6	25.2
Property, plant and equipment – gross	661.8	621.7
Accumulated depreciation	(189.4)	(166.8)
Assets held for sale	0.3	0.0
Property, plant and equipment – net	472.7	454.9
Other Assets
Restricted cash	10.6	10.0
Deferred financing costs	4.5	5.9
Deferred compensation plan assets	7.0	7.3
Other	0.0	0.1
Total	22.1	23.3
Other Accrued Liabilities
Current derivative liabilities – Notes 8 and 9	16.5	94.9
Uncleared cash disbursements	10.1	9.1
Accrued income taxes and taxes payable	6.4	5.2
Accrued annual contribution to VEBAs	0.0	13.7
Accrued contingent contribution to Union VEBA – Note 5	14.2	0.0
Short-term environmental accruals – Note 7	2.2	2.3
Accrued interest	6.2	3.7
Short-term deferred revenue	1.5	0.2
Other	4.3	3.7
Total	61.4	132.8
Long-Term Liabilities
Derivative liabilities – Notes 8 and 9	2.8	1.9
Income tax liabilities	0.7	2.4
Workers’ compensation accruals	20.5	21.5
Long-term environmental accruals – Note 7	16.6	17.0
Long-term asset retirement obligations	4.5	4.4
Deferred compensation liabilities	7.3	7.2
Long-term deferred revenue	0.4	0.5
Long-term capital leases	0.1	0.1
Long-term portion of contingent contribution to Union VEBA – Note 5	29.9	0.0
Other long-term liabilities	3.2	3.3
Total	86.0	58.3
Billed
Trade Receivables – Net
Trade receivables, gross	132.2	128.7
Unbilled
Trade Receivables – Net
Trade receivables, gross	$ 0.2	$ 1.4