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Summary of Significant Accounting Policies - Change in Accounting Principle (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Sep. 30, 2021
Jan. 01, 2019
Dec. 31, 2018
Accounting Policies [Abstract]            
Percentage of LIFO inventory       65.00%    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cost of sales $ 2,466.6 $ 2,689.3 $ 3,484.5      
Gross profit 333.2 292.8 638.0      
Operating Income (Loss) 117.6 (1,302.7) 366.3      
Other income (loss), net 18.2 (1.2) 64.4      
Income (loss) before income taxes 10.6 (1,481.9) 236.5      
Net income (loss) (16.2) (1,559.6) 265.0      
Net loss attributable to ATI $ (38.2) $ (1,572.6) $ 252.5      
Basic net income (loss) attributable to ATI per common share (in dollars per share) $ (0.30) $ (12.43) $ 2.01      
Diluted net income (loss) attributable to ATI per common share (in dollars per share) $ (0.30) $ (12.43) $ 1.81      
Comprehensive income $ 220.5 $ (1,570.1) $ 195.8      
Comprehensive income attributable to ATI 193.7 (1,594.5) 184.6      
Inventories, net 1,046.3 997.1        
Total current assets 2,306.7 2,066.0        
Total assets 4,285.2 4,034.9 5,634.6      
Retained earnings 72.7 106.5       $ 1,427.1
Total ATI stockholders’ equity 685.6 521.1        
Total stockholders’ equity 832.7 641.4 2,193.2     1,991.6
Total liabilities and stockholders’ equity 4,285.2 4,034.9        
Net loss (gain) from sales of businesses (13.8) 0.0 7.1      
Inventories (53.9) $ 158.2 25.2      
Change in Inventory Costing Method, Previous Method            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cost of sales 2,559.2          
Gross profit 240.6          
Operating Income (Loss) 25.0          
Income (loss) before income taxes (82.0)          
Net income (loss) (108.8)          
Net loss attributable to ATI $ (130.8)          
Basic net income (loss) attributable to ATI per common share (in dollars per share) $ (1.03)          
Diluted net income (loss) attributable to ATI per common share (in dollars per share) $ (1.03)          
Comprehensive income $ 127.9          
Comprehensive income attributable to ATI 101.1          
Inventories, net 953.7          
Total current assets 2,214.1          
Total assets 4,192.6          
Retained earnings (19.9)          
Total ATI stockholders’ equity 593.0          
Total stockholders’ equity 740.1          
Total liabilities and stockholders’ equity 4,192.6          
Inventories 38.7          
Change in Inventory Costing Method            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cost of sales (92.6)          
Gross profit 92.6          
Operating Income (Loss) 92.6          
Income (loss) before income taxes 92.6          
Net income (loss) 92.6          
Net loss attributable to ATI $ 92.6          
Basic net income (loss) attributable to ATI per common share (in dollars per share) $ 0.73          
Diluted net income (loss) attributable to ATI per common share (in dollars per share) $ 0.73          
Comprehensive income $ 92.6          
Comprehensive income attributable to ATI 92.6          
Inventories, net 92.6          
Total current assets 92.6          
Total assets 92.6          
Retained earnings 92.6          
Total ATI stockholders’ equity 92.6          
Total stockholders’ equity 92.6          
Total liabilities and stockholders’ equity 92.6          
Inventories $ (92.6)          
Previously Reported            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cost of sales     3,484.7      
Gross profit     637.8      
Operating Income (Loss)     366.1      
Other income (loss), net     69.7      
Income (loss) before income taxes     241.6      
Net income (loss)     270.1      
Net loss attributable to ATI     $ 257.6      
Basic net income (loss) attributable to ATI per common share (in dollars per share)     $ 2.05      
Diluted net income (loss) attributable to ATI per common share (in dollars per share)     $ 1.85      
Comprehensive income     $ 200.9      
Comprehensive income attributable to ATI     189.7      
Retained earnings           $ 1,422.0
Net loss (gain) from sales of businesses     1.8      
Inventories     25.4      
Revision of Prior Period, Change in Accounting Principle, Adjustment | Change in Inventory Costing Method            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cost of sales     (0.2)      
Gross profit     0.2      
Operating Income (Loss)     0.2      
Other income (loss), net     (5.3)      
Income (loss) before income taxes     (5.1)      
Net income (loss)     (5.1)      
Net loss attributable to ATI     $ (5.1)      
Basic net income (loss) attributable to ATI per common share (in dollars per share)     $ (0.04)      
Diluted net income (loss) attributable to ATI per common share (in dollars per share)     $ (0.04)      
Comprehensive income     $ (5.1)      
Comprehensive income attributable to ATI     (5.1)      
Retained earnings         $ 5.1  
Net loss (gain) from sales of businesses     5.3      
Inventories     $ (0.2)