v3.26.3
Related Party Transactions (Cost of Sales from Related Parties) (Details) - Affiliated Entity - Westlake - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2026
Jun. 30, 2026
Mar. 31, 2026
Sep. 30, 2025
Jun. 30, 2025
Mar. 31, 2025
Jun. 30, 2026
Jun. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Related Party Transaction [Line Items]                          
Charges from related parties in cost of sales $ 157,344     $ 148,726         $ 457,644 $ 430,553      
Revision of Prior Period, Error Correction, Adjustment                          
Related Party Transaction [Line Items]                          
Charges from related parties in cost of sales   $ 21,048 $ 22,643   $ 20,415 $ 20,140 $ 43,691 $ 40,555     $ 80,861 $ 71,156 $ 77,570
Feedstock Purchased                          
Related Party Transaction [Line Items]                          
Charges from related parties in cost of sales 112,056     104,753         317,295 298,146      
Feedstock Purchased | Previously Reported                          
Related Party Transaction [Line Items]                          
Charges from related parties in cost of sales       95,110           271,176      
Feedstock Purchased | Revision of Prior Period, Error Correction, Adjustment                          
Related Party Transaction [Line Items]                          
Charges from related parties in cost of sales   10,233 11,350   9,173 8,154 21,583 17,327     37,586 38,269 39,207
Other service charges                          
Related Party Transaction [Line Items]                          
Charges from related parties in cost of sales $ 45,288     43,973         $ 140,349 132,407      
Other service charges | Previously Reported                          
Related Party Transaction [Line Items]                          
Charges from related parties in cost of sales       $ 33,952           $ 99,158      
Other service charges | Revision of Prior Period, Error Correction, Adjustment                          
Related Party Transaction [Line Items]                          
Charges from related parties in cost of sales   $ 10,815 $ 11,293   $ 11,242 $ 11,986 $ 22,108 $ 23,228     $ 43,275 $ 32,887 $ 38,363