v3.26.1
Acquisitions - Dowlais (Details)
$ / shares in Units, shares in Millions, $ in Millions
5 Months Ended 6 Months Ended
Feb. 03, 2026
USD ($)
shares
Jun. 30, 2026
USD ($)
shares
Jun. 30, 2026
USD ($)
shares
Jun. 30, 2025
USD ($)
Feb. 03, 2026
£ / shares
Feb. 03, 2026
USD ($)
$ / shares
shares
Dec. 31, 2025
USD ($)
shares
Oct. 03, 2025
USD ($)
Business Combination [Line Items]                
Common stock, shares authorized | shares   375.0 375.0       150.0  
Goodwill   $ 684.7 $ 684.7       $ 174.4  
6.375% Notes                
Business Combination [Line Items]                
Secured Debt   $ 850.0 $ 850.0       $ 850.0 $ 850.0
6.375% Notes | Secured Debt [Member]                
Business Combination [Line Items]                
Debt Instrument, Interest Rate, Stated Percentage   6.375% 6.375%       6.375% 6.375%
7.750% Notes                
Business Combination [Line Items]                
Unsecured Debt   $ 1,250.0 $ 1,250.0       $ 1,250.0 $ 1,250.0
7.750% Notes | Unsecured Debt [Member]                
Business Combination [Line Items]                
Debt Instrument, Interest Rate, Stated Percentage   7.75% 7.75%       7.75% 7.75%
Tranche C Term Facility                
Business Combination [Line Items]                
Secured Debt   $ 824.6 $ 824.6     $ 835.0 $ 0.0  
Dowlais Group plc                
Business Combination [Line Items]                
Business Combination, Name of Acquiree     Dowlais Group plc          
Business Combination, Common Stock, Conversion Ratio           0.0881    
Business Combination, Price Per Share | (per share)         £ 43 $ 0.59    
Business Combination, Consideration Transferred, Equity Interest, Share Issued, Number of Shares | shares 117.0              
Common stock, shares authorized | shares           375.0    
Business Combination, Consideration Transferred $ 1,724.7              
Payments to Acquire Businesses, Gross $ 791.3              
Business Combination, Consideration Transferred, Equity Interest, Share Issued, Value           $ 933.4    
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value           19.3    
Business Combination, Total Fair Value Acquired           1,744.0    
Business Combination, Recognized Asset Acquired, Cash and Cash Equivalent           459.7    
Business Combination, Recognized Asset Acquired, Receivable, Current           656.8    
Business Combination, Recognized Asset Acquired, Inventory, Current           538.4    
Business Combination, Recognized Asset Acquired, Property, Plant, and Equipment           2,706.4    
Business Combination, Recognized Asset Acquired, Investments in Equity-Method Affiliates           883.7    
Business Combination, Recognized Asset Acquired, Prepaid Expenses and Other Long-Term Assets           569.1    
Business Combination, Recognized Asset Acquired, Asset           5,814.1    
Business Combination, Recognized Liability Assumed, Accounts Payable, Current           784.1    
Business Combination, Recognized Liability Assumed, Other Liability, Current           785.6    
Business Combination, Recognized Liability Assumed, Deferred Tax Liability           340.5    
Business Combination, Recognized Liability Assumed, Long-Term Debt, Noncurrent           1,876.9    
Business Combination, Recognized Liability Assumed, Other Liability, Noncurrent           794.0    
Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less)           1,233.0    
Goodwill           511.0    
Business Combination, Acquiree's Revenue since Acquisition Date, Actual   2,411.0            
Business Combination, Acquiree's Earnings (Loss) since Acquisition Date, Actual   50.0            
Fair Value Inventory Adjustment from Acquisition of Dowlais, Net of Tax   $ 30.0            
Business Combination, Pro Forma Information, Pro Forma Revenue     $ 5,825.8 $ 5,749.2        
Business Combination, Pro Forma Information, Pro Forma Income (Loss), after Tax     (32.4) (92.5)        
Business Combination, Pro Forma, Nonrecurring Adjustment, Net of Tax     73.0          
Dowlais Group plc | Deferred income tax liabilities                
Business Combination [Line Items]                
Goodwill, Measurement Period Adjustment     43.0          
Dowlais Group plc | Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment                
Business Combination [Line Items]                
Business Combination, Pro Forma, Nonrecurring Adjustment     38.0          
Dowlais Group plc | Business Combination, Pro Forma Information, Nonrecurring Adjustment, Acquisition-Related Cost                
Business Combination [Line Items]                
Business Combination, Pro Forma, Nonrecurring Adjustment     $ 54.0          
Dowlais Group plc | Tranche C Term Facility                
Business Combination [Line Items]                
Secured Debt       $ 843.0   $ 835.0