Fair Value Measurements (Summary of Carrying and Fair Values of Long Term Debt) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, carrying value	$ 477,608	$ 473,960
OpCo Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, fair value	230,649	228,180
OpCo Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member] | Limited Partner [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, carrying value	224,064	220,416
MLP Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, fair value	256,588	258,234
MLP Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member] | Limited Partner [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt payable to Westlake, carrying value	$ 253,544	$ 253,544