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Debt - Additional Information (Detail)
¥ in Millions
1 Months Ended 12 Months Ended
Feb. 15, 2022
Jan. 25, 2017
USD ($)
Feb. 28, 2018
USD ($)
Feb. 28, 2017
USD ($)
Dec. 31, 2017
USD ($)
Installment
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2017
CNY (¥)
Installment
Debt Instrument [Line Items]                
Loss on extinguishment of debt       $ 10,000,000 $ 13,300,000 $ 0 $ 0  
Bond premium and bond tender expenses       7,300,000        
Write-off unamortized debt issuance cost       2,700,000        
Accumulated amortization         1,800,000 12,000,000    
Unamortized debt issuance costs         $ 11,700,000 $ 8,700,000    
6.00 percent Senior Notes due 2025 [Member] | Senior Notes [Member]                
Debt Instrument [Line Items]                
Senior notes, offering value   $ 500,000,000            
Senior notes, effective percentage   6.00%            
Senior notes, payment terms         The 2025 Notes pay interest semi-annually in arrears on February 15 and August 15 beginning on August 15, 2017 and will mature on February 15, 2025 unless earlier redeemed or repurchased.      
Debt instrument, redemption price percentage         104.50%      
Debt maturity date         Feb. 15, 2025      
6.00 percent Senior Notes due 2025 [Member] | Senior Notes [Member] | Scenario, Forecast [Member]                
Debt Instrument [Line Items]                
Debt instrument, redemption price percentage 101.50%              
Senior Secured Credit Facilities [Member]                
Debt Instrument [Line Items]                
Letters of credit, amount outstanding         $ 41,700,000      
Committed Loan Facility Agreements [Member] | Construction Loans [Member]                
Debt Instrument [Line Items]                
Debt maturity date         Dec. 21, 2020      
Number of installments for repayment of loans | Installment         6     6
Installment frequency for repayment of loans         6 months      
Commencement period of debt repayment         2018-06      
Committed Loan Facility Agreements [Member] | Construction Loans [Member] | Third Party Debt [Member]                
Debt Instrument [Line Items]                
Construction loan facility borrowing capacity | ¥               ¥ 198.8
Committed Loan Facility Agreements [Member] | Construction Loans [Member] | Committed Credit Facility [Member] | Lender [Member]                
Debt Instrument [Line Items]                
Construction loan facility borrowing capacity | ¥               66.2
Committed Loan Facility Agreements [Member] | Construction Loans [Member] | Koppers Carbon and Chemical Company Limited [Member]                
Debt Instrument [Line Items]                
Percentage ownership of subsidiary         75.00%      
Construction loan facility borrowing capacity         $ 44,000,000     ¥ 265.0
Percentage of non-controlling shareholders         25.00%     25.00%
Revolving Credit Facility [Member]                
Debt Instrument [Line Items]                
Revolving credit facility       $ 400,000,000        
Revolving credit facility, variable interest rate basis       LIBOR        
Revolving credit facility, maturity period       2022-02        
Write-off unamortized debt issuance cost       $ 3,300,000        
Revolving Credit Facility [Member] | Senior Secured Credit Facilities [Member]                
Debt Instrument [Line Items]                
Debt instrument, unused borrowing capacity         $ 203,300,000      
Subsequent Event [Member] | Revolving Credit Facility [Member]                
Debt Instrument [Line Items]                
Revolving credit facility     $ 600,000,000          
Revolving credit facility, variable interest rate basis     LIBOR