Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Cash and Cash Equivalents
Cash and money market funds	$ 42.3	$ 29.5
Commercial paper	12.0	148.2
Total	54.3	177.7
Trade Receivables – Net
Trade receivables, gross	139.4	130.1
Allowance for doubtful receivables	(0.8)	(0.8)
Trade receivables – net	138.6	129.3
Inventories
Finished products	64.6	73.6
Work-in-process	70.2	66.7
Raw materials	60.4	54.2
Operating supplies and repair and maintenance parts	22.1	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	2.0	0.0
Other	5.6	6.2
Total	100.4	178.6
Property, Plant and Equipment – Net
Land and improvements	22.7	22.9
Buildings and leasehold improvements	64.1	63.8
Machinery and equipment	523.2	509.8
Construction in progress	36.1	25.2
Property, plant and equipment – gross	646.1	621.7
Accumulated depreciation	(181.9)	(166.8)
Assets held for sale	0.3	0.0
Property, plant and equipment – net	464.5	454.9
Other Assets
Restricted cash	10.4	10.0
Deferred financing costs	5.0	5.9
Deferred compensation plan assets	7.6	7.3
Other	0.1	0.1
Total	23.1	23.3
Other Accrued Liabilities
Current derivative liabilities – Notes 8 and 9	15.6	94.9
Uncleared cash disbursements	5.0	9.1
Accrued income taxes and taxes payable	6.3	5.2
Accrued annual contribution to VEBAs	0.0	13.7
Accrued contingent contribution to Union VEBA - Note 5	17.1	0.0
Short-term environmental accruals – Note 7	2.2	2.3
Accrued interest	1.7	3.7
Short-term deferred revenue	0.2	0.2
Other	3.5	3.7
Total	51.6	132.8
Long-Term Liabilities
Derivative liabilities – Notes 8 and 9	2.0	1.9
Income tax liabilities	0.7	2.4
Workers’ compensation accruals	20.7	21.5
Long-term environmental accruals – Note 7	17.3	17.0
Long-term asset retirement obligations	4.7	4.4
Deferred compensation liabilities	7.8	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	86.8	58.3
Billed
Trade Receivables – Net
Trade receivables, gross	138.6	128.7
Unbilled
Trade Receivables – Net
Trade receivables, gross	$ 0.8	$ 1.4