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Segment Information (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2025
USD ($)
segment
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Segment Reporting Information [Line Items]      
Number of operating segments | segment 1    
Number of reportable segments | segment 1    
Depreciation and amortization $ 127,978 $ 111,899 $ 110,203
Cost of sales 818,847 716,957 803,332
Selling, general and administrative expenses 28,271 28,495 29,751
Interest expense—Westlake 22,899 25,701 26,501
Net income 298,576 369,159 334,626
Assets 1,256,533 1,287,956  
Operating Segments | OpCo      
Segment Reporting Information [Line Items]      
Raw material, energy, manufacturing and logistics costs 690,869 605,058 693,129
Depreciation and amortization 127,978 111,899 110,203
Cost of sales 818,847 716,957 803,332
Selling, general and administrative expenses 24,853 24,699 26,217
Other segment items (582) (3,005) (1,786)
Interest expense—Westlake 1,316 1,410 1,496
Net income 323,577 397,245 363,028
Assets 1,222,123 1,238,420  
Corporate      
Segment Reporting Information [Line Items]      
Interest expense—Westlake 21,583 24,291 25,005
Net income (25,001) (28,086) (28,402)
Assets 34,410 49,536  
Affiliated Entity      
Segment Reporting Information [Line Items]      
Net co-products, ethylene and other sales—third parties $ 1,033,276 $ 950,801 $ 1,026,655