Supplemental Balance Sheet Information (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash and Cash Equivalents
Total	$ 177.7	$ 169.5	$ 273.4	$ 49.8	$ 49.8
Trade Receivables - Net
Billed trade receivables	128.7	120.2
Unbilled trade receivables — Note 1	1.4	0.4
Trade receivables, gross	130.1	120.6
Allowance for doubtful receivables	(0.8)	(0.8)
Trade receivables - net	129.3	119.8
Inventories
Finished products	73.6	72.5
Work-in-process	66.7	75.9
Raw materials	54.2	47.2
Operating supplies and repair and maintenance parts	20.2	18.8
Total	214.7	214.4
Prepaid Expenses and Other Current Assets
Current derivative assets — Notes 10 and 11	85.7	2.0
Current deferred tax assets	86.4	36.7
Short-term restricted cash	0.3	0.3
Other	6.2	5.2
Total	178.6	44.2
Property, Plant and Equipment - Net
Land and improvements	22.9	22.6
Buildings and leasehold improvements	63.8	53.0
Machinery and equipment	509.8	425.6
Construction in progress	25.2	66.0
Property, plant and equipment - gross	621.7	567.2
Accumulated depreciation	(166.8)	(137.9)
Property, plant and equipment - net	454.9	429.3
Other Assets
Derivative assets — Notes 10 and 11	0	79.8
Restricted cash	10.0	9.3
Deferred financing costs	5.9	8.9
Deferred compensation plan assets	7.3	6.5
Other	0.1	0.3
Total	23.3	104.8
Other Accrued Liabilities
Current derivative liabilities — Notes 10 and 11	94.9	1.8
Uncleared cash disbursements	9.1	9.6
Accrued income taxes and taxes payable	5.2	4.3
Accrued annual VEBA contribution	13.7	16.0
Short-term environmental accrual — Note 9	2.3	2.8
Accrued interest	3.7	3.7
Short-term deferred revenue — Note 1	0.2	0
Other	3.7	6.6
Total	132.8	44.8
Long-Term Liabilities
Derivative liabilities — Notes 10 and 11	1.9	84.3
Income tax liabilities	2.4	5.0
Workers’ compensation accruals	21.5	23.3
Long-term environmental accrual — Note 9	17.0	20.0
Long-term asset retirement obligations	4.4	4.0
Long-term deferred revenue - Note 1	0.5	0
Deferred compensation liability	7.2	7.0
Long-term capital leases	0.1	0.1
Other long-term liabilities	3.3	2.7
Total	58.3	146.4
Long-Term Debt — Note 3
Senior notes	225.0	225.0
Cash convertible senior notes	0	163.5
Total	225.0	388.5
Fair Value, Measurements, Recurring [Member]
Other Assets
Deferred compensation plan assets	7.3	6.5
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Cash and Cash Equivalents
Cash and money market funds	29.5	57.7
Other Assets
Deferred compensation plan assets	0	0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Cash and Cash Equivalents
Commercial paper	148.2	111.8
Other Assets
Deferred compensation plan assets	$ 7.3	$ 6.5