Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Sales	$ 692.5	$ 955.5
Segment EBITDA	74.3	117.7
LIFO and net realizable value reserves	0.0	0.0
Corporate expenses	(12.2)	(12.3)
Closed operations and other income (expense)	0.5	(6.3)
Total ATI Adjusted EBITDA	62.6	99.1
Depreciation & amortization	(36.1)	(37.3)
Interest expense, net	(23.4)	(21.9)
Restructuring and other charges	0.0	(8.0)
Gain on asset sales	0.0	2.5
Income before income taxes	3.1	34.4
Operating Segments
Segment Reporting Information [Line Items]
Sales	741.2	1,036.1
Depreciation & amortization	(36.1)	(37.3)
Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	265.2	448.5
Segment EBITDA	24.6	76.6
Depreciation & amortization	(19.6)	(19.5)
Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	476.0	587.6
Segment EBITDA	49.7	41.1
Depreciation & amortization	(15.5)	(17.0)
Operating Segments | Corporate Segment
Segment Reporting Information [Line Items]
Depreciation & amortization	(1.0)	(0.8)
Internal Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	48.7	80.6
Internal Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	24.3	28.2
Internal Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	24.4	52.4
External Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	692.5	955.5
External Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	240.9	420.3
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	$ 451.6	$ 535.2