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Joint Ventures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 141.6	$ 229.6	$ 149.8	$ 269.5
Related party accounts receivable	1.2	1.5
Undistributed earnings of investees accounted for under equity method	$ 10.0
Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%
Cash and cash equivalents	$ 26.2
Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%
Cash and cash equivalents	$ 7.0
Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%
Revenue from related parties	$ 38.6	20.3	55.4
China Baowu Steel Group Corporation Limited [Member] | Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	40.00%
VSMPO [Member] | Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%
Flat Rolled Products [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	$ 0.6	$ 0.5	$ (0.1)

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