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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		10 Months Ended	12 Months Ended			34 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Dec. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Sale to noncontrolling interest				$ 0.0	$ 14.4	$ 3.7
Gain on joint venture deconsolidation		$ 15.9		0.0	15.9	0.0
Cash and cash equivalents	$ 490.8		$ 382.0	490.8	382.0	141.6		$ 229.6
Income (loss) from equity method investments				(10.7)	(1.0)	0.0
Intercompany notes receivable	0.0		0.0	0.0	0.0
Undistributed earnings of investees accounted for under equity method	14.5			14.5
Non-cash joint venture impairment charge	11.4			11.4	0.0	0.0
Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	22.9			22.9
Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Sale to noncontrolling interest					2.7
Cash and cash equivalents	$ 9.9			$ 9.9
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%			50.00%
Impairment of long-lived assets held-for-use				$ 14.2
Income (loss) from equity method investments				(19.3)	(3.9)
Revenue from related parties				14.6	4.1
Related party accounts receivable	$ 0.1		0.6	0.1	0.6
Due from affiliates, allowance for credit loss	$ 4.3			4.3
Non-cash joint venture impairment charge				$ 11.4
Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%			50.00%
Revenue from related parties				$ 31.3	49.4	38.6
Related party accounts receivable	$ 0.2		1.8	0.2	1.8
Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments				(12.2)
Due from affiliates	36.8		10.5	36.8	10.5
Advanced Alloys & Solutions \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments				1.5	2.9	$ 0.6
Prepaid expenses and other current assets [Member] \| Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates	8.3			8.3
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates	$ 28.5		10.5	$ 28.5	$ 10.5
Allegheny Technologies Inc \| Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%			60.00%
Allegheny Technologies Inc \| Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%			51.00%
Allegheny Technologies Inc \| 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%			40.00%
Allegheny Technologies Inc \| VSMPO [Member] \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%			50.00%
Property, Plant and Equipment \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments				$ (7.1)
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sale to noncontrolling interest			$ 12.0
Equity method investment ownership percentage	50.00%			50.00%
Forecast [Member] \| Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sale to noncontrolling interest							$ 17.5