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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		17 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Aug. 06, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 281.2	$ 122.4		$ 281.2	$ 122.4		$ 382.0	$ 141.6
Sales to noncontrolling interests				0.0	14.4
Gain on joint venture deconsolidation	0.0	0.0		0.0	15.9
Income (loss) from equity method investments	(3.5)	2.3		(6.3)	2.4
Investment in subsidiaries	$ 0.0			$ 0.0			$ 0.0
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%			51.00%
Cash and cash equivalents	$ 6.9			$ 6.9
Sales to noncontrolling interests			$ 2.7
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests					12.0
Equity method investment ownership percentage	50.00%			50.00%
Uniti
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%			50.00%
Tsingshan Group | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%			50.00%
VSMPO | Uniti
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%			50.00%
Flat Rolled Products | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%			60.00%
Cash and cash equivalents	$ 45.5			$ 45.5
Flat Rolled Products | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	(4.0)	1.1		(7.3)	0.5
Investment in subsidiaries	3.7			3.7
Due from Affiliates	43.8			43.8
Flat Rolled Products | Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	$ 0.5	$ 1.3		$ 1.0	$ 1.8
Flat Rolled Products | China Baowu Steel Group Corporation Limited | Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	40.00%			40.00%
Forecast | Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests						$ 17.5

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