Business Segments (Details) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Abstract]
Number of business segments \| segment	2
Segment Reporting Information [Line Items]
Sales	$ 979.0	$ 865.9
Operating profit (loss)	96.4	69.9
LIFO and net realizable value reserves	0.0	0.0
Corporate expenses	(13.2)	(10.3)
Closed operations and other expenses	(8.1)	(3.0)
Gain on joint venture deconsolidation (See Note 5)	15.9	0.0
Interest expense, net	(25.5)	(33.5)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	65.5	23.1
Operating Segments
Segment Reporting Information [Line Items]
Sales	1,018.5	896.7
Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	579.4	523.7
Operating profit (loss)	85.5	50.9
Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	439.1	373.0
Operating profit (loss)	10.9	19.0
Internal Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	39.5	30.8
Internal Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	18.7	13.3
Internal Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	20.8	17.5
External Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	979.0	865.9
External Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	560.7	510.4
External Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	$ 418.3	$ 355.5

corporate office expenses not allocable to business segments

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Lifo Provision Benefit and Change in Debit LIFO NRV reserve, net

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Business segment operating profit, which excludes costs for restructuring charges, retirement benefit income or expense, corproate expenses, interest expense, debt extinguishment costs and costs associated with closed operations. These costs are excluded for segment reporting to provide a profit measure based on what management considers to be controllable costs at the business segment level

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The amount of the gain (loss) recognized by the parent and included in its attributable portion of net income for the period due to deconsolidation of a subsidiary or derecognition of a group of assets. The gain (loss) recognized and included in the net income attributable to the parent for the period is generally computed as the difference between: (a) the aggregate of: (1) the fair value of any consideration received; (2) the fair value of any retained noncontrolling investment in the former subsidiary at the date the subsidiary was deconsolidated; and (3) the carrying amount of any noncontrolling interest in the former subsidiary (including any accumulated other comprehensive income attributable to the noncontrolling interest) at the date the subsidiary was deconsolidated and (b) the carrying amount of the former subsidiary's assets and liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84161450&loc=SL4582445-111684

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=68072869&loc=d3e41242-110953

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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-Name Accounting Standards Codification
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Aggregate revenue during the period from the sale of goods in the normal course of business, after deducting returns, allowances and discounts.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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