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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents		$ 122.4		$ 154.6	$ 122.4	$ 154.6		$ 141.6	$ 229.6
Sales to noncontrolling interests					14.4	2.2
Gain on joint venture deconsolidation (See Note 5)		$ 0.0		0.0	$ 15.9	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		$ 11.4			$ 11.4
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		$ 52.7			$ 52.7
Flat Rolled Products \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments		1.1			0.5
Flat Rolled Products \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments		$ 1.3		$ 0.0	$ 1.8	$ 0.2
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	$ 5.5						$ 17.5