Long-term Debt Payable to Westlake (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Long-term debt payable to Westlake	$ 477,608	$ 473,960
Weighted average interest rate	4.08%	3.76%
Limited Partner [Member] | August 2013 Promissory Note [Member] | Senior Notes [Member] | Prime Rate [Member]
Related Party Transaction [Line Items]
Basis spread on variable rate, percent	1.50%	1.50%
Limited Partner [Member] | OpCo Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Long-term debt payable to Westlake	$ 224,064	$ 220,416
Limited Partner [Member] | OpCo Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member] | LIBOR [Member]
Related Party Transaction [Line Items]
Basis spread on variable rate, percent	3.00%	3.00%
Limited Partner [Member] | MLP Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Long-term debt payable to Westlake	$ 253,544	$ 253,544
Limited Partner [Member] | MLP Revolver [Member] | Senior Unsecured Revolving Credit Facility [Member] | LIBOR [Member]
Related Party Transaction [Line Items]
Basis spread on variable rate, percent	2.00%	2.00%