Restructuring Costs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) employee	Dec. 31, 2020 USD ($) position	Dec. 31, 2019 USD ($) position
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ (10.5)	$ 1,132.1	$ 4.5
Non-cash restructuring charges	0.0	1,041.5	0.0
Restructuring charges (credits)	(11.3)	1,107.5	4.5
Severance costs		$ 60.5
Restructuring and Related Cost, Expected Number of Positions Eliminated | position		1,400
Inventory write-down	0.8	$ 7.2
Termination benefits		17.4
Brackenridge PA Operations
Restructuring Cost and Reserve [Line Items]
Asset impairment charges		1,032.6
Fair value		$ 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
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
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 (credits)		5.5
Restructuring Reserve [Roll Forward]
Restructuring Charges	$ 0.7
Employee Severance
Restructuring Cost and Reserve [Line Items]
Reduction in severance related reserves, workforce | employee	350
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	$ 43.4	4.5	0.0
Restructuring Charges	(12.0)	60.5	4.5
Payments for Restructuring	(13.7)	(21.6)	0.0
Restructuring Reserve, Ending Balance	17.7	43.4	$ 4.5
Other Current Liabilities [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	33.8
Restructuring Reserve, Ending Balance	11.4	33.8
Other Noncurrent Liabilities [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	9.6
Restructuring Reserve, Ending Balance	$ 6.3	$ 9.6
Flat Rolled Products
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Number of Positions Eliminated | position			70