Business Segments (Details8) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Reconciling Item For Operating Profit Loss From Segment To Consolidated [Line Items]
Inventory valuation reserves	$ 65,400,000	$ 82,800,000
Segment EBITDA	351,600,000	244,600,000	$ 528,800,000
Corporate expenses	(55,900,000)	(40,900,000)	(65,300,000)
Closed operations and other expenses	(4,800,000)	(7,400,000)	(24,000,000.0)
Total ATI Adjusted EBITDA	290,900,000	196,300,000	439,500,000
Depreciation and amortization	(143,900,000)	(143,300,000)	(151,100,000)
Interest expense, net	(96,900,000)	(94,400,000)	(99,000,000.0)
Restructuring and other charges	(10,500,000)	1,132,100,000	4,500,000
Strike related costs	(63,200,000)	0	0
Settlement gain	64,900,000	0	0
Impairment of goodwill	0	(287,000,000)	0
Joint venture restructuring and impairment charges	0	(2,400,000)	(11,400,000)
Debt extinguishment charge	(65,500,000)	(21,500,000)	(21,600,000)
Gain on asset sales and sale of business, net	13,800,000	2,500,000	84,600,000
Income (loss) before income taxes	10,600,000	(1,481,900,000)	236,500,000
High Performance Materials & Components
Segment Reporting Reconciling Item For Operating Profit Loss From Segment To Consolidated [Line Items]
Impairment of goodwill	0
Operating Segments
Segment Reporting Reconciling Item For Operating Profit Loss From Segment To Consolidated [Line Items]
Depreciation and amortization	(143,900,000)	(143,300,000)	(151,100,000)
Operating Segments | High Performance Materials & Components
Segment Reporting Reconciling Item For Operating Profit Loss From Segment To Consolidated [Line Items]
Segment EBITDA	159,900,000	129,600,000	356,200,000
Depreciation and amortization	(75,000,000.0)	(78,100,000)	(84,600,000)
Operating Segments | Advanced Alloys & Solutions [Member]
Segment Reporting Reconciling Item For Operating Profit Loss From Segment To Consolidated [Line Items]
Segment EBITDA	191,700,000	115,000,000.0	172,600,000
Depreciation and amortization	$ (64,500,000)	$ (62,100,000)	$ (63,500,000)