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Business Segments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Restructuring charges			$ 527.2	$ 64.3	$ 0.0
Total direct international sales			1,277.1	1,577.0	1,607.5
Sales by U.S. operations to foreign countries			971.4	1,215.8	1,201.8
LIFO-related net realizable value charge		$ (136.4)	(97.3)	(136.4)
Inventory valuation reserves		206.3	169.0	206.3
Closed operations and other expenses			34.6	22.1	28.3
Closed operations environmental costs			5.8	4.5	8.0
Closed operations insurance obligations					3.8
Other corporate expense			23.8	15.3	9.4
Flat Rolled Products [Member]
Segment Reporting Information [Line Items]
Increase (Decrease) in Assets			(32.0)
Income (loss) from equity method investments			0.5	(0.1)	(8.9)
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			$ 20.3	55.4	75.3
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			$ 5.0	2.3	$ 7.1
Inventory Valuation Reserve [Member] \| Titanium Sponge Inventory [Member]
Segment Reporting Information [Line Items]
Inventory valuation charges	$ 11.3		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