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Related Party Transactions
 12 Months Ended
Dec. 31, 2025
Related Party Transactions [Abstract]  
Related Party Transactions Related Party Transactions
Related party transactions between the Partnership, OpCo and Westlake are primarily based on agreements such as the Ethylene Sales Agreement, the Feedstock Supply Agreement, the Services and Secondment Agreement, Site Lease Agreements, the Omnibus Agreement, the Investment Management Agreement, the Exchange Agreement, the OpCo Partnership Agreement, the OpCo Revolver and the MLP Revolver. These agreements, discussed in detail in Notes 2 and 8, reflect the pervasive effect of the relationship with Westlake on the Partnership's operations and its consolidated financial statements. Pursuant to these agreements, the Partnership and OpCo regularly enter into transactions with Westlake. See below for descriptions and details of significant related party transactions.
Sales to Related Parties
OpCo sells ethylene to Westlake under the Ethylene Sales Agreement. Additionally, the Partnership and OpCo from time to time provide other services or products for which it charges Westlake a fee.
OpCo sells a significant portion of its ethylene production to Westlake. Sales to related parties were as follows:
Year Ended December 31,
202520242023
Net sales—Westlake$1,033,276 $950,801 $1,026,655 
As of December 31, 2025, OpCo had an annual production deficiency under the Ethylene Sales Agreement for the full year 2025 primarily due to the Petro 1 turnaround extending into April 2025, which was later than the planned completion in March 2025. The total Buyer Deficiency Fee recognized by the Partnership during the twelve months ended December 31, 2025 was $5,835. The Buyer Deficiency Fee is measured periodically based upon the lower of the actual production deficiency at period end or the estimated annual production deficiency based upon OpCo's annual anticipated production. These periodic estimates are updated at the end of the year based on actual annual production. The Buyer Deficiency Fee is classified as a component of net sales—Westlake.
Cost of Sales from Related Parties
Charges for goods and services purchased by the Partnership and OpCo from Westlake and included in cost of sales relate primarily to feedstock purchased under the Feedstock Supply Agreement and services provided under the Services and Secondment Agreement.
A significant portion of the Partnership's inputs included in cost of sales are supplied by Westlake. Charges from related parties for significant inputs included in cost of sales were as follows:
Year Ended December 31,
202520242023
Feedstock purchased from Westlake and included in cost of sales$382,698 $310,856 $398,700 
Other charges from Westlake and included in cost of sales133,824 125,702 123,039 
Services from Related Parties Included in Selling, General and Administrative Expenses
Charges for services purchased by the Partnership from Westlake and included in selling, general and administrative expenses primarily relate to services Westlake performs on behalf of the Partnership under the Omnibus Agreement, including the Partnership's finance, legal, information technology, human resources, communication, ethics and compliance and other administrative functions.
Charges from related parties included within selling, general and administrative expenses were as follows:
Year Ended December 31,
202520242023
Services received from Westlake and included in selling, general and
   administrative expenses		$25,017 $25,654 $27,085 
Goods and Services from Related Parties Capitalized as Assets
Charges for goods and services purchased by the Partnership and OpCo from Westlake which were capitalized as assets relate primarily to the services of Westlake employees under the Services and Secondment Agreement.
Charges from related parties for goods and services capitalized as assets were as follows:
Year Ended December 31,
202520242023
Goods and services purchased from Westlake and capitalized as assets$7,311 $2,295 $3,660 
Receivable under the Investment Management Agreement
On August 1, 2017, the Partnership, OpCo and Westlake executed an investment management agreement (the "Investment Management Agreement") that authorized Westlake to invest the Partnership's and OpCo's excess cash with Westlake for durations of up to a maximum of nine months. Per the terms of the Investment Management Agreement, the Partnership earns a market return plus five basis points and Westlake provides daily availability of the invested cash to meet any liquidity needs of the Partnership or OpCo. Accrued interest of $206 and $1,385 was included in the receivable under the Investment Management Agreement balance at December 31, 2025 and 2024, respectively. Total interest earned related to the Investment Management Agreement was $2,539, $5,364 and $4,424 for the years ended December 31, 2025, 2024 and 2023, respectively.
The Partnership's receivable under the Investment Management Agreement was as follows:
December 31,
20252024
Receivable under the Investment Management Agreement$23,378 $134,557 
Accounts Receivable from Related Parties
The Partnership's accounts receivable from Westlake result primarily from ethylene sales to Westlake and any Buyer Deficiency Fee and Shortfall recognized under the Ethylene Sales Agreement.
The Buyer Deficiency Fee of $5,835, as discussed above under "Sales to Related Parties" recognized in the twelve months ended December 31, 2025, was received by the Partnership in January 2026.
As a result of the Partnership's decision to postpone the maintenance turnaround at the Petro 1 facility from the third quarter of 2024 to the first quarter of 2025, OpCo's production resulted in "excess quantities" of ethylene for the contract year ended December 31, 2024. Pursuant to the terms of the Ethylene Sales Agreement, the excess quantities produced were sold to Westlake at prices that excluded certain non-variable costs of production. As of December 31, 2024, accounts receivable included sales associated with the excess quantities, which were at a lower sales price.
The Partnership's accounts receivable from Westlake were as follows:
December 31,
20252024
Accounts receivable—Westlake$63,571 $31,975 
Accounts Payable to Related Parties
The Partnership's accounts payable to Westlake result primarily from feedstock purchases under the Feedstock Supply Agreement and services provided under the Services and Secondment Agreement and the Omnibus Agreement.
The Partnership's accounts payable to Westlake were as follows:
December 31,
20252024
Accounts payable—Westlake$19,132 $20,744 
Debt Payable to Related Parties
See Note 8 for a description of related party debt payable balances.
Interest on related party debt payable balances, net of capitalized interest, for the years ended December 31, 2025, 2024 and 2023 was $22,899, $25,701 and $26,501, respectively. Interest on related party debt payable is presented as interest expense—Westlake in the consolidated statements of operations. At December 31, 2025 and 2024, accrued interest on related party debt was $5,508 and $5,848, respectively, and is reflected as a component of accrued and other liabilities in the consolidated balance sheets.
Debt payable to related parties was as follows:
December 31,
20252024
Long-term debt payable to Westlake$399,674 $399,674 
Related Party Leases
OpCo is obligated to Westlake under various long-term and short-term noncancelable operating leases, primarily related to rail cars and land. Operating lease rentals paid to Westlake for such leases were $1,705, $1,843 and $1,869 for the years ended December 31, 2025, 2024 and 2023, respectively, and are reflected in other charges from Westlake that are included in cost of sales.
OpCo has two site lease agreements with Westlake, each of which has a term of 50 years. Pursuant to the site lease agreements, OpCo pays Westlake one dollar per site per year.
Major Customer and Concentration of Credit Risk
During the years ended December 31, 2025, 2024 and 2023, Westlake accounted for approximately 88.6%, 83.7% and 86.2%, respectively, of the Partnership's net sales.
General
During the years ended December 31, 2025, 2024 and 2023, the Partnership reimbursed $333, $351 and $347, respectively, to Westlake for certain state tax payments.
Other
See Note 10 above for an additional transaction through which a related party acquired common units in a private placement.