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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 109.9	$ 159.8			$ 141.6	$ 229.6
Sales to noncontrolling interests	7.4	0.0
Gain on joint venture deconsolidation (See Note 5)	$ 15.9	0.0
Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%
Cash and cash equivalents	$ 12.0
Sales to noncontrolling interests	2.7
Allegheny & Tsingshan Stainless [Member]
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests	$ 5.0
Equity method investment ownership percentage	50.00%
Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%
Tsingshan Group [Member] \| Allegheny & Tsingshan Stainless [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%
VSMPO [Member] \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%
Flat Rolled Products \| 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	$ 40.8
Flat Rolled Products \| Allegheny & Tsingshan Stainless [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	(0.6)
Flat Rolled Products \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	$ 0.5	$ 0.2
Flat Rolled Products \| 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%
Forecast \| Allegheny & Tsingshan Stainless [Member]
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests			$ 12.5	$ 17.5