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Business Segments - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
Business Segments - Narrative (Details) $ in Millions	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			2
Restructuring charges			$ 0.0	$ 527.2	$ 64.3
Total international sales			1,454.5	1,277.1	1,577.0
Sales by U.S. operations to foreign countries			1,078.6	971.4	1,215.8
LIFO-related net realizable value charge			(43.1)	(97.3)
Inventory valuation reserves			121.5	169.0
Closed operations and other expenses			34.0	34.6	22.1
Closed operations environmental costs			2.9	5.8	4.5
Decrease in employee benefit liabilities			(3.7)
Other corporate expense			22.7	23.8	15.3
Foreign currency remeasurement impacts			7.3
Flat Rolled Products [Member]
Segment Reporting Information [Line Items]
Increase (Decrease) in Assets				(32.0)
Income (loss) from equity method investments			$ 0.6	0.5	(0.1)
High Performance Materials & Components [Member]
Segment Reporting Information [Line Items]
Increase (Decrease) in Assets				(521.0)
STAL Precision Stainless Steel Company Limited [Member]
Segment Reporting Information [Line Items]
Joint venture ownership percentage			60.00%
Uniti [Member]
Segment Reporting Information [Line Items]
Equity method investment ownership percentage			50.00%
Revenue from related parties			$ 38.6	20.3	55.4
Uniti [Member] \| VSMPO [Member]
Segment Reporting Information [Line Items]
Joint venture ownership percentage by unaffiliated entity			50.00%
Corporate, Non-Segment [Member] \| Closed Company and Other [Member]
Segment Reporting Information [Line Items]
Retirement Benefit Expense			$ 4.8	5.0	2.3
Inventory Valuation Reserve [Member] \| Titanium sponge inventory [Member]
Segment Reporting Information [Line Items]
Inventory valuation charges	$ 11.3
Inventory Valuation Reserve [Member] \| Non-PQ grade titanium sponge [Member]
Segment Reporting Information [Line Items]
Inventory valuation charges		$ 25.4
Employee benefit costs [Member]
Segment Reporting Information [Line Items]
Restructuring charges				24.2
Employee benefit costs [Member] \| Flat Rolled Products [Member]
Segment Reporting Information [Line Items]
Restructuring charges				4.9
Employee benefit costs [Member] \| High Performance Materials & Components [Member]
Segment Reporting Information [Line Items]
Restructuring charges				7.5
Long-lived asset impairment [Member]
Segment Reporting Information [Line Items]
Restructuring charges				471.3
Asset Impairment Charges					$ 54.5
Facility Closing [Member]
Segment Reporting Information [Line Items]
Restructuring charges		$ 3.5		31.7
Facility Closing [Member] \| Flat Rolled Products [Member]
Segment Reporting Information [Line Items]
Restructuring charges				8.4
Facility Closing [Member] \| High Performance Materials & Components [Member]
Segment Reporting Information [Line Items]
Restructuring charges				$ 23.8
Aerospace & Defense Market Concentration Risk [Member] \| Sales Revenue, Net [Member] \| High Performance Materials & Components [Member]
Segment Reporting Information [Line Items]
Percentage of sales			75.00%
Oil & Gas and Automotive Market Concentration [Member] \| Sales Revenue, Net [Member] \| Flat Rolled Products [Member]
Segment Reporting Information [Line Items]
Percentage of sales			40.00%