Supplemental Balance Sheet Information (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Cash and Cash Equivalents.
Cash and money market funds		$ 57.7
Commercial paper		111.8
Total	212.3	169.5	224.6	273.4
Trade Receivables - Net.
Trade receivables, gross	133.2	120.6
Allowance for doubtful receivables	(0.8)	(0.8)
Trade receivables - net	132.4	119.8
Inventories.
Finished products	64.9	72.5
Work-in-process	70.5	75.9
Raw materials	56.0	47.2
Operating supplies and repair and maintenance parts	19.9	18.8
Total	211.3	214.4
Prepaid Expenses and Other Current Assets.
Current derivative assets – Notes 8 and 9	101.6	2.0
Current deferred tax assets	47.8	36.7
Short-term restricted cash	0.3	0.3
Prepaid taxes	1.0	0
Other	5.7	5.2
Total	156.4	44.2
Property, Plant and Equipment - Net.
Land and improvements	22.8	22.6
Buildings and leasehold improvements	62.4	53.0
Machinery and equipment	500.0	425.6
Construction in progress	18.1	66.0
Active property, plant and equipment, gross	603.3	567.2
Accumulated depreciation	(160.4)	(137.9)
Property, plant and equipment - net	442.9	429.3
Other Assets.
Derivative assets – Notes 8 and 9	0.2	79.8
Restricted cash	10.0	9.3
Deferred financing costs	6.3	8.9
Deferred compensation plan assets	7.2	6.5
Other	0.1	0.3
Total	23.8	104.8
Other Accrued Liabilities.
Current derivative liabilities – Notes 8 and 9	101.9	1.8
Uncleared cash disbursements	6.6	9.6
Accrued income taxes and taxes payable	6.3	4.3
Accrued annual VEBA contribution	0	16.0
Short-term environmental accrual – Note 7	3.3	2.8
Accrued interest	10.3	3.7
Deferred Revenue, Current	0.2	0
Other	3.1	6.6
Total	131.7	44.8
Long-Term Liabilities.
Derivative liabilities – Notes 8 and 9	0.6	84.3
Income tax liabilities	5.0	5.0
Workers’ compensation accruals	23.0	23.3
Long-term environmental accrual – Note 7	16.9	20.0
Long-term asset retirement obligations	4.3	4.0
Deferred compensation liability	7.6	7.0
Deferred Revenue, Noncurrent	0.6	0
Long-term capital leases	0.1	0.1
Other long-term liabilities	2.8	2.7
Total	60.9	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 - Net.
Trade receivables, gross	132.2	120.2
Unbilled
Trade Receivables - Net.
Trade receivables, gross	1.0	0.4
Recurring
Cash and Cash Equivalents.
Cash and cash equivalents	212.3	169.5
Other Assets.
Deferred compensation plan assets	7.2	6.5
Fair Value, Inputs, Level 1 [Member] | Recurring
Cash and Cash Equivalents.
Cash and cash equivalents	35.4	57.7
Other Assets.
Deferred compensation plan assets	0	0
Level 2 | Recurring
Cash and Cash Equivalents.
Cash and cash equivalents	176.9	111.8
Other Assets.
Deferred compensation plan assets		$ 6.5