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Accumulated Other Comprehensive Income (Loss) (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]    
Beginning balance $ 5,860  
Ending balance 5,949  
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]    
Increase (Decrease) in Stockholders' Equity [Roll Forward]    
Beginning balance 3 $ 27
Other comprehensive loss before reclassifications 0 0
Net other comprehensive loss 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 loss before reclassifications 5 (4)
Net other comprehensive loss attributable to Westlake Chemical Corporation 5 (4)
Ending balance (72) (93)
Accumulated Other Comprehensive Income (Loss) [Member]    
Increase (Decrease) in Stockholders' Equity [Roll Forward]    
Beginning balance (74) (62)
Other comprehensive loss before reclassifications 5 (4)
Net other comprehensive loss attributable to Westlake Chemical Corporation 5 (4)
Ending balance $ (69) $ (66)