Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Noncontrolling Interest in OpCo [Member]	Accumulated Other Comprehensive Income [Member]	Westlake [Member] General Partner [Member]	Common units [Member] Public [Member] Limited Partner [Member]	Common units [Member] Westlake [Member] Limited Partner [Member]	Subordinated units [Member] Westlake [Member] Limited Partner [Member]
Equity, beginning balance at Dec. 31, 2016	$ 920,963	$ 818,479	$ 200	$ (242,430)	$ 297,367	$ 4,813	$ 42,534
Stockholders' Equity [Roll Forward]
Net income	353,058	304,388	0	1,666	23,743	9,934	13,327
Net effect of cash flow hedge	79		79
Proceeds from secondary public offering, net of finance and other offering costs	110,698				110,698
Subordinated unit conversion	0					42,352	(42,352)
Quarterly distribution to unitholders	(42,117)	0	0	(1,194)	(20,580)	(6,834)	(13,509)
Quarterly distribution to noncontrolling interest retained in OpCo by Westlake	(343,932)	(343,932)	0	0	0	0	0
Equity, ending balance at Dec. 31, 2017	998,749	778,935	279	(241,958)	411,228	50,265	0
Stockholders' Equity [Roll Forward]
Net income	330,571	281,224	0	733	27,320	21,294	0
Net effect of cash flow hedge	(279)		(279)
Units issued for vested phantom units	291				291
Proceeds from secondary public offering, net of finance and other offering costs	0
Quarterly distribution to unitholders	(53,363)	0	0	(1,347)	(29,231)	(22,785)	0
Quarterly distribution to noncontrolling interest retained in OpCo by Westlake	(341,888)	(341,888)	0	0	0	0	0
Equity, ending balance at Dec. 31, 2018	934,081	718,271	0	(242,572)	409,608	48,774	0
Stockholders' Equity [Roll Forward]
Net income	332,895	271,914	0	0	35,978	25,003	0
Units issued for vested phantom units	146				146
Proceeds from secondary public offering, net of finance and other offering costs	62,661		0		62,661
Quarterly distribution to unitholders	(62,084)	0	0	0	(36,657)	(25,427)	0
Quarterly distribution to noncontrolling interest retained in OpCo by Westlake	(315,564)	(315,564)	0	0	0	0	0
Equity, ending balance at Dec. 31, 2019	$ 952,135	$ 674,621	$ 0	$ (242,572)	$ 471,736	$ 48,350	$ 0