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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		24 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 153.5		$ 124.9	$ 153.5	$ 124.9		$ 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	$ 10.4			$ 10.4
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	$ 30.0			$ 30.0
Flat Rolled Products \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	(0.3)			0.2
Flat Rolled Products \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	$ 0.5		$ (0.1)	$ 2.3	$ 0.1
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