Accumulated Other Comprehensive Income (Loss) (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 5,860
Ending balance	5,901
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	3	$ 27
Other comprehensive income before reclassifications	0	0
Net other comprehensive income attributable to Westlake Chemical Corporation	0	0
Ending balance	3	27
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(77)	(89)
Other comprehensive income before reclassifications	(13)	12
Net other comprehensive income attributable to Westlake Chemical Corporation	(13)	12
Ending balance	(90)	(77)
Accumulated Other Comprehensive Income (Loss) [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(74)	(62)
Other comprehensive income before reclassifications	(13)	12
Net other comprehensive income attributable to Westlake Chemical Corporation	(13)	12
Ending balance	$ (87)	$ (50)