Partners' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 29, 2017	Aug. 04, 2014	Jun. 30, 2018	Dec. 31, 2017	Aug. 30, 2017
Class of Stock [Line Items]
Proceeds from secondary public offering, net of finance and other offering costs	$ 110,739
Underwriting discounts, structuring fees and offering expenses on issuance of equity	$ 3,111
Westlake [Member] | Subordinated units [Member]
Class of Stock [Line Items]
Units outstanding			0	0	12,686,115
Limited Partner [Member]
Class of Stock [Line Items]
Number of units sold in public offering	5,175,000	12,937,500
Units sold in secondary offering price per unit (in dollars per share)	$ 22.00