Supplemental Balance Sheet Information (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Cash and Cash Equivalents.
Cash and money market funds	$ 68.5	$ 57.7
Commercial paper	117.1	111.8
Total	185.6	169.5	248.0	273.4
Trade Receivables.
Trade receivables, gross	124.3	120.6
Allowance for doubtful receivables	(0.8)	(0.8)
Trade receivables, net	123.5	119.8
Inventories.
Finished products	65.9	72.5
Work-in-process	66.9	75.9
Raw materials	51.5	47.2
Operating supplies and repair and maintenance parts	18.8	18.8
Total	203.1	214.4
Prepaid Expenses and Other Current Assets.
Current derivative assets – Notes 8 and 9	4.1	2.0
Current deferred tax assets	36.7	36.7
Short-term restricted cash	0.3	0.3
Prepaid taxes	2.8	0
Prepaid expenses	4.7	5.2
Total	48.6	44.2
Property, Plant and Equipment - Net.
Land and improvements	22.8	22.6
Buildings and leasehold improvements	54.2	53.0
Machinery and equipment	455.3	425.6
Construction in progress	49.3	66.0
Property, plant and equipment, gross	581.6	567.2
Accumulated depreciation	(144.9)	(137.9)
Total	436.7	429.3
Other Assets.
Derivative assets – Notes 8 and 9	84.2	79.8
Restricted cash	9.3	9.3
Deferred financing costs	8.2	8.9
Deferred compensation plan assets	6.6	6.5
Other	0.2	0.3
Total	108.5	104.8
Other Accrued Liabilities.
Current derivative liabilities – Notes 8 and 9	2.0	1.8
Uncleared cash disbursements	10.5	9.6
Accrued income taxes and taxes payable	7.6	4.3
Accrued annual VEBA contribution	0	16.0
Short-term environmental accrual – Note 7	3.6	2.8
Accrued interest	10.3	3.7
Other	3.1	6.6
Total	37.1	44.8
Long-term Liabilities.
Derivative liabilities – Notes 8 and 9	87.7	84.3
Income tax liabilities	4.9	5.0
Workers’ compensation accruals	23.4	23.3
Long-term environmental accrual – Note 7	18.9	20.0
Long-term asset retirement obligations	4.1	4.0
Deferred compensation liability	7.2	7.0
Long-term capital lease	0.1	0.1
Other long-term liabilities	2.7	2.7
Total	149.0	146.4
Long-term Debt. — Note 3
Senior notes	225.0	225.0
Cash convertible senior notes	165.7	163.5
Total	390.7	388.5
Billed
Trade Receivables.
Trade receivables, gross	124.1	120.2
Unbilled
Trade Receivables.
Trade receivables, gross	$ 0.2	$ 0.4