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Restructuring Costs (Details) $ in Millions	3 Months Ended					12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) position	Dec. 31, 2019 USD ($) position	Dec. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 1,105.1	$ 2.3	$ 16.7	$ 8.0	$ 4.5	$ 1,132.1	$ 4.5	$ 0.0
Non-cash restructuring charges	1,041.5					1,041.5	0.0	0.0
Restructuring charges						1,107.5	4.5	0.0
Severance costs	33.5					$ 60.5
Restructuring and Related Cost, Expected Number of Positions Eliminated | position						1,400
Inventory write-down						$ 7.2
Termination benefits	17.4					$ 17.4
Stainless Steel Sheet Products
Restructuring Cost and Reserve [Line Items]
Planned Period Until Production Ceased						1 year
Brackenridge PA Operations
Restructuring Cost and Reserve [Line Items]
Asset impairment charges						$ 1,032.6
Fair value	$ 354.0					$ 354.0
Brackenridge PA Operations | Weighted average cost of capital | Fair Value, Inputs, Level 3 [Member]
Restructuring Cost and Reserve [Line Items]
Long-Lived Asset, Measurement Input	0.093					0.093
Brackenridge PA Operations | Long-term growth rate | Fair Value, Inputs, Level 3 [Member]
Restructuring Cost and Reserve [Line Items]
Long-Lived Asset, Measurement Input	0.02					0.02
Various AA&S Segment Operations
Restructuring Cost and Reserve [Line Items]
Asset impairment charges						$ 8.9
Facility Idling
Restructuring Cost and Reserve [Line Items]
Restructuring charges						5.5
Employee Severance
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance				$ 4.5		4.5	0.0
Restructuring Charges						60.5	4.5
Payments for Restructuring						(21.6)	0.0
Restructuring Reserve, Ending Balance	$ 43.4				$ 4.5	43.4	$ 4.5	$ 0.0
Other Current Liabilities [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Ending Balance	33.8					33.8
Other Noncurrent Liabilities [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Ending Balance	$ 9.6					$ 9.6
Flat Rolled Products
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Number of Positions Eliminated | position							70

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