Stock-based Compensation - Additional Information (Details) - shares	3 Months Ended
	Mar. 31, 2026	Jan. 31, 2026	Dec. 31, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	1,199,427		1,264,468
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	679,416		734,231
Stock-based awards, vesting period	3 years
Vesting performance stock units if minimum performance criteria are not achieved (in shares)	0
Performance Shares, Granted 2025 And Prior
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, term in years	3 years
Performance Shares, Granted in 2026
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target award from cumulative adjusted earning per share	50.00%
Percentage of target award from cumulative free cash flow	50.00%
Adjusted EBITDA margin, performance modifier, percentage	25.00%
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	520,011		530,237
Weighted Average \| Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding		137,844
Weighted Average \| Market Condition Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding		124,878
Minimum \| Performance Shares, Granted 2025 And Prior
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target award earned by participants	0.00%
Minimum \| Performance Shares, Granted in 2026
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target award earned by participants	0.00%
Minimum \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based awards, vesting period	3 years
Maximum \| Performance Shares, Granted 2025 And Prior
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target award earned by participants	200.00%
Maximum \| Performance Shares, Granted in 2026
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target award earned by participants	250.00%
Maximum \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based awards, vesting period	4 years

Koppers Holdings Inc. 10-Q