Supplemental Balance Sheet Information (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Cash and Cash Equivalents
Cash and money market funds	$ 204.9	$ 29.5
Commercial paper	16.0	148.2
Total	220.9	177.7	185.6	169.5
Trade Receivables – Net
Trade receivables, gross	150.5	130.1
Allowance for doubtful receivables	(0.8)	(0.8)
Trade receivables – net	149.7	129.3
Inventories
Finished products	69.8	73.6
Work-in-process	71.5	66.7
Raw materials	55.6	54.2
Operating supplies and repair and maintenance parts	21.1	20.2
Total	218.0	214.7
Prepaid Expenses and Other Current Assets
Current derivative assets – Notes 8 and 9	95.1	85.7
Current deferred tax assets	92.2	86.4
Short-term restricted cash	0.3	0.3
Prepaid taxes	3.0	0
Other	5.2	6.2
Total	195.8	178.6
Property, Plant and Equipment – Net
Land and improvements	22.9	22.9
Buildings and leasehold improvements	64.2	63.8
Machinery and equipment	518.0	509.8
Construction in progress	27.4	25.2
Property, plant and equipment – gross	632.5	621.7
Accumulated depreciation	(174.4)	(166.8)
Property, plant and equipment – net	458.1	454.9
Other Assets
Restricted cash	10.4	10.0
Deferred financing costs	5.4	5.9
Deferred compensation plan assets	7.1	7.3
Other	0.1	0.1
Total	23.0	23.3
Other Accrued Liabilities
Current derivative liabilities – Notes 8 and 9	108.1	94.9
Uncleared cash disbursements	9.2	9.1
Accrued income taxes and taxes payable	8.2	5.2
Accrued annual contribution to VEBAs	0	13.7
Accrued contingent contribution to Union VEBA - Note 5	15.5	0
Short-term environmental accruals – Note 7	2.2	2.3
Accrued interest	10.3	3.7
Short-term deferred revenue	0.2	0.2
Other	3.5	3.7
Total	157.2	132.8
Long-Term Liabilities
Derivative liabilities – Notes 8 and 9	2.6	1.9
Income tax liabilities	2.3	2.4
Workers’ compensation accruals	21.0	21.5
Long-term environmental accruals – Note 7	17.2	17.0
Long-term asset retirement obligations	4.6	4.4
Deferred compensation liabilities	7.6	7.2
Long-term deferred revenue	0.5	0.5
Long-term capital leases	0.1	0.1
Long-term portion of contingent contribution to Union VEBA - Note 5	29.9	0
Other long-term liabilities	3.3	3.3
Total	89.1	58.3
Billed
Trade Receivables – Net
Trade receivables, gross	149.9	128.7
Unbilled
Trade Receivables – Net
Trade receivables, gross	$ 0.6	$ 1.4