v3.26.1
Derivative Financial Instruments - Summary of Fair Value of Outstanding Derivative Contracts Recorded in Balance Sheet (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Derivative [Line Items]    
Net asset (liability) on balance sheet $ 21.0 $ 27.8
Accumulated other comprehensive gain, net of tax 2.6 0.1
Balance Sheet Location: us-gaap:AccruedLiabilitiesCurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet (0.9) (2.6)
Balance Sheet Location: us-gaap:OtherAssetsNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.4  
Balance Sheet Location: us-gaap:OtherLiabilitiesNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet (1.7) (1.1)
Balance Sheet Location: kop:DerivativeFairValueOfDerivativeCurrentAssets    
Derivative [Line Items]    
Net asset (liability) on balance sheet 23.2 31.5
Copper Swap Contracts    
Derivative [Line Items]    
Net asset (liability) on balance sheet 20.2 31.4
Accumulated other comprehensive gain, net of tax 1.7 2.5
Copper Swap Contracts | Balance Sheet Location: us-gaap:AccruedLiabilitiesCurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.0 0.0
Copper Swap Contracts | Balance Sheet Location: us-gaap:OtherAssetsNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.0  
Copper Swap Contracts | Balance Sheet Location: us-gaap:OtherLiabilitiesNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet (0.6) 0.0
Copper Swap Contracts | Balance Sheet Location: kop:DerivativeFairValueOfDerivativeCurrentAssets    
Derivative [Line Items]    
Net asset (liability) on balance sheet 20.8 31.4
Heating Oil Contracts    
Derivative [Line Items]    
Net asset (liability) on balance sheet 2.1 (0.4)
Accumulated other comprehensive gain, net of tax 0.0 0.0
Heating Oil Contracts | Balance Sheet Location: us-gaap:AccruedLiabilitiesCurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.0 (0.4)
Heating Oil Contracts | Balance Sheet Location: us-gaap:OtherAssetsNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.4  
Heating Oil Contracts | Balance Sheet Location: us-gaap:OtherLiabilitiesNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.0 0.0
Heating Oil Contracts | Balance Sheet Location: kop:DerivativeFairValueOfDerivativeCurrentAssets    
Derivative [Line Items]    
Net asset (liability) on balance sheet 1.7 0.0
Foreign Currency Forward Contracts    
Derivative [Line Items]    
Net asset (liability) on balance sheet (0.1) 0.0
Accumulated other comprehensive gain, net of tax 0.0 0.0
Foreign Currency Forward Contracts | Balance Sheet Location: us-gaap:AccruedLiabilitiesCurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet (0.1) 0.0
Foreign Currency Forward Contracts | Balance Sheet Location: us-gaap:OtherAssetsNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.0  
Foreign Currency Forward Contracts | Balance Sheet Location: us-gaap:OtherLiabilitiesNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.0 0.0
Foreign Currency Forward Contracts | Balance Sheet Location: kop:DerivativeFairValueOfDerivativeCurrentAssets    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.0 0.0
Interest Rate Swap Contracts    
Derivative [Line Items]    
Net asset (liability) on balance sheet (1.2) (3.2)
Accumulated other comprehensive gain, net of tax 0.9 (2.4)
Interest Rate Swap Contracts | Balance Sheet Location: us-gaap:AccruedLiabilitiesCurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet (0.8) (2.2)
Interest Rate Swap Contracts | Balance Sheet Location: us-gaap:OtherAssetsNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet 0.0  
Interest Rate Swap Contracts | Balance Sheet Location: us-gaap:OtherLiabilitiesNoncurrent    
Derivative [Line Items]    
Net asset (liability) on balance sheet (1.1) (1.1)
Interest Rate Swap Contracts | Balance Sheet Location: kop:DerivativeFairValueOfDerivativeCurrentAssets    
Derivative [Line Items]    
Net asset (liability) on balance sheet $ 0.7 $ 0.1