Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Sales	$ 616.2	$ 770.3	$ 1,308.7	$ 1,725.8
Segment EBITDA	73.2	62.1	147.5	179.8
LIFO and net realizable value reserves	0.0	0.0	0.0	0.0
Corporate expenses	(15.9)	(7.2)	(28.1)	(19.5)
Closed operations and other income (expense)	(3.6)	2.7	(3.1)	(3.6)
Total ATI Adjusted EBITDA	53.7	57.6	116.3	156.7
Depreciation & amortization	(36.3)	(35.6)	(72.4)	(72.9)
Interest expense, net	(23.7)	(21.7)	(47.1)	(43.6)
Restructuring charges (credits)	(6.2)	16.7	(6.2)	24.7
Strike related costs	(40.3)	0.0	(40.3)	0.0
Impairment of goodwill	0.0	(287.0)	0.0	(287.0)
Income (loss) from equity investments	0.0	(1.9)	0.0	(5.6)
Debt extinguishment charge (See Note 7)	0.0	(21.5)	0.0	(21.5)
Gain on asset sales	0.0	0.0	0.0	2.5
Loss before income taxes	(40.4)	(327.3)	(37.3)	(292.9)
Employee Severance [Member]
Segment Reporting Information [Line Items]
Restructuring charges (credits)	(6.9)		(6.9)
Allegheny & Tsingshan Stainless | Employee Severance [Member]
Segment Reporting Information [Line Items]
Income (loss) from equity investments	0.0	(2.4)	0.0	(2.4)
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless
Segment Reporting Information [Line Items]
Income (loss) from equity investments	(0.4)	(4.6)	(0.9)	(8.5)
Operating Segments
Segment Reporting Information [Line Items]
Sales	658.1	822.9	1,399.3	1,859.0
Depreciation & amortization	(36.3)	(35.6)	(72.4)	(72.9)
Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	318.7	316.7	583.9	765.2
Segment EBITDA	37.2	28.7	61.8	105.3
Depreciation & amortization	(19.2)	(19.5)	(38.8)	(39.0)
Strike related costs	(2.1)
Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	339.4	506.2	815.4	1,093.8
Segment EBITDA	36.0	33.4	85.7	74.5
Depreciation & amortization	(16.1)	(15.3)	(31.6)	(32.3)
Strike related costs	(38.2)
Operating Segments | Corporate Segment
Segment Reporting Information [Line Items]
Depreciation & amortization	(1.0)	(0.8)	(2.0)	(1.6)
Internal Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	41.9	52.6	90.6	133.2
Internal Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	18.1	16.0	42.4	44.2
Internal Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	23.8	36.6	48.2	89.0
External Customers | Operating Segments
Segment Reporting Information [Line Items]
Sales	616.2	770.3	1,308.7	1,725.8
External Customers | Operating Segments | High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	300.6	300.7	541.5	721.0
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Information [Line Items]
Sales	$ 315.6	$ 469.6	$ 767.2	$ 1,004.8