Supplemental Balance Sheet Information (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Cash and Cash Equivalents.
Cash and money market funds	$ 69.7	$ 57.7
Commercial paper	104.5	111.8
Total	174.2	169.5	281.7	273.4
Trade Receivables.
Trade receivables, gross	123.9	120.6
Allowance for doubtful receivables	(0.8)	(0.8)
Trade receivables, net	123.1	119.8
Inventories.
Finished products	60.4	72.5
Work-in-process	64.3	75.9
Raw materials	53.3	47.2
Operating supplies and repair and maintenance parts	20.1	18.8
Total	198.1	214.4
Prepaid Expenses and Other Current Assets.
Current derivative assets – Notes 8 and 9	90.3	2.0
Current deferred tax assets	36.7	36.7
Short-term restricted cash	0.3	0.3
Prepaid taxes	1.8	0
Other	6.4	5.2
Total	135.5	44.2
Property, Plant and Equipment - Net.
Land and improvements	22.8	22.6
Buildings and leasehold improvements	54.7	53.0
Machinery and equipment	491.2	425.6
Construction in progress	24.7	66.0
Active property, plant and equipment, gross	593.4	567.2
Accumulated depreciation	(152.0)	(137.9)
Total	441.4	429.3
Other Assets.
Derivative assets – Notes 8 and 9	0.3	79.8
Restricted cash	9.3	9.3
Deferred financing costs	6.7	8.9
Deferred compensation plan assets	7.4	6.5
Other	0.3	0.3
Total	24.0	104.8
Other Accrued Liabilities.
Current derivative liabilities – Notes 8 and 9	89.5	1.8
Uncleared cash disbursements	9.1	9.6
Accrued income taxes and taxes payable	6.3	4.6
Accrued annual VEBA contribution	0	16.0
Short-term environmental accrual – Note 7	3.6	2.8
Accrued interest	3.6	3.7
Other	3.0	6.3
Total	115.1	44.8
Long-Term Liabilities.
Derivative liabilities – Notes 8 and 9	0.5	84.3
Income tax liabilities	5.2	5.0
Workers’ compensation accruals	24.4	23.3
Long-term environmental accrual – Note 7	18.6	20.0
Long-term asset retirement obligations	4.3	4.0
Deferred compensation liability	7.7	7.0
Long-term capital lease	0.1	0.1
Other long-term liabilities	2.9	2.7
Total	63.7	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
Billed
Trade Receivables.
Trade receivables, gross	123.7	120.2
Unbilled
Trade Receivables.
Trade receivables, gross	$ 0.2	$ 0.4