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Supplemental Information (Restructuring Accrual Activity) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2026	Jun. 30, 2026	Mar. 31, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2025	Dec. 31, 2024
Restructuring Reserve [Roll Forward]
Charges	$ 58			$ 17	$ 79	$ 139
Pernis Facility Closure
Restructuring Reserve [Roll Forward]
Balances at beginning of period	175	$ 198	$ 226		226
Charges	(3)	(4)	4	$ 15		$ 130	$ 247	$ 75
Payments for Restructuring	(11)	(17)	(28)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)	0	(2)	(4)
Balances at end of period	161	175	198		161		226
Cumulative charges recognized	319				319
Pernis Facility Closure \| Inventory Write-Down [Member]
Restructuring Reserve [Roll Forward]
Charges							15
Pernis Facility Closure \| Asset Retirement Obligations [Member]
Restructuring Reserve [Roll Forward]
Balances at beginning of period	73	88	99		99
Charges	(4)	(3)	4
Payments for Restructuring	(7)	(11)	(13)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)	0	(1)	(2)
Balances at end of period	62	73	88		62		99
Cumulative charges recognized	119				119
Pernis Facility Closure \| Employee Severance [Member]
Restructuring Reserve [Roll Forward]
Balances at beginning of period	14	16	25		25
Charges	1	1	2
Payments for Restructuring	(1)	(2)	(11)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)	0	(1)	0
Balances at end of period	14	14	16		14		25
Cumulative charges recognized	31				31
Expected restructuring charges remaining	3				3
Pernis Facility Closure \| Facility Closing [Member]
Restructuring Reserve [Roll Forward]
Balances at beginning of period	88	94	102		102
Charges	0	(2)	(2)
Payments for Restructuring	(3)	(4)	(4)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)	0	0	(2)
Balances at end of period	85	88	94		85		102
Cumulative charges recognized	154				154
Pernis Facility Closure \| Asset Write-Downs
Restructuring Reserve [Roll Forward]
Cumulative charges recognized	15				15
Chlorovinyls and Styrene Facility Closure
Restructuring Reserve [Roll Forward]
Balances at beginning of period	26	43	76		76
Charges	7	7	11				393
Payments for Restructuring	(17)	(24)	(44)
Balances at end of period	14	26	43		14		76
Cumulative charges recognized	418				418
Chlorovinyls and Styrene Facility Closure \| Inventory Write-Down [Member]
Restructuring Reserve [Roll Forward]
Charges							30
Chlorovinyls and Styrene Facility Closure \| Asset Retirement Obligations [Member]
Restructuring Reserve [Roll Forward]
Balances at beginning of period	20	34	52		52
Charges	3	2	0
Payments for Restructuring	(12)	(16)	(18)
Balances at end of period	11	20	34		11		52
Cumulative charges recognized	57				57
Chlorovinyls and Styrene Facility Closure \| Employee Severance [Member]
Restructuring Reserve [Roll Forward]
Balances at beginning of period	4	4	17		17
Charges	0	1	3
Payments for Restructuring	(2)	(1)	(16)
Balances at end of period	2	4	4		2		17
Cumulative charges recognized	21				21
Expected restructuring charges remaining	1				1
Chlorovinyls and Styrene Facility Closure \| Facility Closing [Member]
Restructuring Reserve [Roll Forward]
Expected restructuring charges remaining	2				2
Chlorovinyls and Styrene Facility Closure \| Other Plant Shutdown Costs
Restructuring Reserve [Roll Forward]
Balances at beginning of period	2	5	7		7
Charges	2	4	8
Payments for Restructuring	(3)	(7)	(10)
Balances at end of period	1	$ 2	$ 5		1		7
Cumulative charges recognized	21				21
Chlorovinyls and Styrene Facility Closure \| Accelerated Depreciation and Amortization
Restructuring Reserve [Roll Forward]
Charges							$ 287
Chlorovinyls and Styrene Facility Closure \| Asset Write-Downs
Restructuring Reserve [Roll Forward]
Charges	2				2
Inventory Write-down	(2)
Chlorovinyls and Styrene Facility Closure \| Asset Write-Downs and Accelerated Depreciation and Amortization
Restructuring Reserve [Roll Forward]
Charges	2
Balances at end of period	0				0
Cumulative charges recognized	319				319
Cologne Facility Closure
Restructuring Reserve [Roll Forward]
Charges					51
Property, Plant and Equipment, net	(33)				(33)
Inventory Write-down					(1)
Asset Retirement Obligation	16				16
Balances at end of period	33				33
Expected restructuring charges remaining	150				150
Cologne Facility Closure \| Asset Retirement Obligations [Member]
Restructuring Reserve [Roll Forward]
Charges					0
Property, Plant and Equipment, net	0				0
Inventory Write-down					0
Asset Retirement Obligation	16				16
Balances at end of period	16				16
Cologne Facility Closure \| Employee Severance [Member]
Restructuring Reserve [Roll Forward]
Charges					17
Property, Plant and Equipment, net	0				0
Inventory Write-down					0
Asset Retirement Obligation	0				0
Balances at end of period	17				17
Cologne Facility Closure \| Facility Closing [Member]
Restructuring Reserve [Roll Forward]
Expected restructuring charges remaining	9				9
Cologne Facility Closure \| Accelerated Depreciation and Amortization
Restructuring Reserve [Roll Forward]
Charges					3
Property, Plant and Equipment, net	(3)				(3)
Inventory Write-down					0
Asset Retirement Obligation	0				0
Balances at end of period	0				0
Cologne Facility Closure \| Restructuring, Transaction and Integration Costs
Restructuring Reserve [Roll Forward]
Charges					50
Cologne Facility Closure \| Cost of Sales
Restructuring Reserve [Roll Forward]
Charges					1
Cologne Facility Closure \| Asset Write-Downs
Restructuring Reserve [Roll Forward]
Charges					31
Property, Plant and Equipment, net	(30)				(30)
Inventory Write-down					(1)
Asset Retirement Obligation	0				0
Balances at end of period	0				0
Cologne Facility Closure \| Asset Write-Downs and Accelerated Depreciation and Amortization
Restructuring Reserve [Roll Forward]
Expected restructuring charges remaining	84				84
Cologne Facility Closure \| Contract Termination Costs
Restructuring Reserve [Roll Forward]
Expected restructuring charges remaining	$ 57				$ 57