Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Sales	$ 725.7	$ 598.0	$ 2,034.4	$ 2,323.8
Segment EBITDA	94.2	27.8	241.7	207.6
LIFO and net realizable value reserves	0.0	0.0	0.0	0.0
Corporate expenses	(12.9)	(10.2)	(41.0)	(29.7)
Closed operations and other expense	(1.4)	(1.0)	(4.5)	(4.6)
Total ATI Adjusted EBITDA	79.9	16.6	196.2	173.3
Depreciation & amortization	(35.6)	(35.4)	(108.0)	(108.3)
Interest expense, net	(25.1)	(25.1)	(72.2)	(68.7)
Restructuring charges (credits)	(2.3)	2.3	(8.5)	27.0
Strike related costs	(22.9)	0.0	(63.2)	0.0
Retirement benefit settlement gain	64.9	0.0	64.9	0.0
Impairment of goodwill	0.0	0.0	0.0	(287.0)
Income (loss) from equity investments	0.4	(1.0)	0.4	(6.6)
Debt extinguishment charge	0.0	0.0	0.0	(21.5)
Gain on asset sales and sale of business	13.7	0.0	13.7	2.5
Income (loss) before income taxes	77.2	(46.2)	39.9	(339.1)
Employee Severance [Member]
Segment Reporting Information [Line Items]
Restructuring charges (credits)			(9.2)
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity investments	0.0	0.0	0.0	(2.4)
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Segment Reporting Information [Line Items]
Income (loss) from equity investments	0.3	(1.6)	(0.6)	(10.1)
Operating Segments
Segment Reporting Information [Line Items]
Sales	789.7	623.4	2,189.0	2,482.4
Depreciation & amortization	(35.6)	(35.4)	(108.0)	(108.3)
Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	325.0	230.1	908.9	995.3
Segment EBITDA	37.4	16.8	99.2	122.1
Depreciation & amortization	(18.2)	(19.5)	(57.0)	(58.5)
Strike related costs	(1.4)		(3.5)
Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	464.7	393.3	1,280.1	1,487.1
Segment EBITDA	56.8	11.0	142.5	85.5
Depreciation & amortization	(16.3)	(15.1)	(47.9)	(47.4)
Strike related costs	(21.5)		(59.7)
Operating Segments | Corporate Segment
Segment Reporting Information [Line Items]
Depreciation & amortization	(1.1)	(0.8)	(3.1)	(2.4)
Internal Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	64.0	25.4	154.6	158.6
Internal Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	25.0	8.8	67.4	53.0
Internal Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	39.0	16.6	87.2	105.6
External Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	725.7	598.0	2,034.4	2,323.8
External Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	300.0	221.3	841.5	942.3
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	$ 425.7	$ 376.7	$ 1,192.9	$ 1,381.5