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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended	24 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 217.0	$ 109.9	$ 382.0		$ 141.6
Sales to noncontrolling interests	0.0	7.4
Gain on joint venture deconsolidation	0.0	15.9
Investment in subsidiaries	$ 0.0		0.0
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%
Cash and cash equivalents	$ 9.0
Sales to noncontrolling interests		2.7
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests		5.0	$ 12.0
Equity method investment ownership percentage	50.00%
Uniti
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%
VSMPO \| Uniti
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
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%
Cash and cash equivalents	$ 39.4
Flat Rolled Products \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	(3.3)	(0.6)
Investment in subsidiaries	7.8
Flat Rolled Products \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments	$ 0.5	$ 0.5
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%
Forecast \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests				$ 17.5