Long-term Debt Payable to Westlake (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Related Party Transaction [Line Items]
Long-term debt payable to Westlake	$ 399,674	$ 399,674
Weighted average interest rate	2.10%	2.20%
Limited Partner \| OpCo Revolver \| Senior Unsecured Revolving Credit Facility
Related Party Transaction [Line Items]
Long-term debt payable to Westlake	$ 22,619	$ 22,619
Limited Partner \| OpCo Revolver \| Senior Unsecured Revolving Credit Facility \| LIBOR [Member]
Related Party Transaction [Line Items]
Basis spread on variable rate, percent	2.00%
Limited Partner \| MLP Revolver \| Senior Unsecured Revolving Credit Facility
Related Party Transaction [Line Items]
Long-term debt payable to Westlake	$ 377,055	$ 377,055
Limited Partner \| MLP Revolver \| Senior Unsecured Revolving Credit Facility \| LIBOR [Member]
Related Party Transaction [Line Items]
Basis spread on variable rate, percent	2.00%

Portion of the carrying amount as of the balance sheet date of obligations due all related parties that is payable after one year or beyond the normal operating cycle if longer.

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+ Details