v3.26.3
Accumulated Other Comprehensive Loss (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($)
$ in Millions
 3 Months Ended
Sep. 30, 2026
Jun. 30, 2026
Mar. 31, 2026
Sep. 30, 2025
Jun. 30, 2025
Mar. 31, 2025
AOCI Attributable to Parent, Net of Tax [Roll Forward]            
Beginning balances $ 9,208 $ 9,051 $ 9,295 $ 10,791 $ 10,921 $ 11,043
Ending balances 9,165 9,208 9,051 9,938 10,791 10,921
Accumulated Other Comprehensive Loss            
AOCI Attributable to Parent, Net of Tax [Roll Forward]            
Beginning balances (116) (99) (80) (64) (130) (144)
Net other comprehensive income (loss) attributable to Westlake Corporation 2 (17) (19) (13) 66 14
Ending balances (114) (116) (99) (77) (64) (130)
Pension and Other Post-Retirement Benefits Liability, Net of Tax            
AOCI Attributable to Parent, Net of Tax [Roll Forward]            
Beginning balances 44 44 44 55 54 54
Net other comprehensive income (loss) attributable to Westlake Corporation 1 0 0 0 1 0
Ending balances 45 44 44 55 55 54
Cumulative Foreign Currency Exchange, Net of Tax            
AOCI Attributable to Parent, Net of Tax [Roll Forward]            
Beginning balances (160) (143) (125) (119) (184) (198)
Net other comprehensive income (loss) attributable to Westlake Corporation 3 (17) (18) (14) 65 14
Ending balances (157) (160) (143) (133) (119) (184)
Available-for-sale Securities, Net of Tax            
AOCI Attributable to Parent, Net of Tax [Roll Forward]            
Beginning balances 0 0 1 0 0 0
Net other comprehensive income (loss) attributable to Westlake Corporation (2) 0 (1) 1 0 0
Ending balances $ (2) $ 0 $ 0 $ 1 $ 0 $ 0