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Long-Term Debt (Schedule Of Long-Term Debt) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017	Nov. 30, 2017	Dec. 31, 2016	Aug. 10, 2016	Jul. 31, 2012	Dec. 31, 2010
Debt Instrument [Line Items]
Principal amount		$ 3,853		$ 3,678
Unamortized Premium (Discount) and Debt Issuance Costs		(16)		1
Long-term debt, net		3,837		3,679
Long-Term Debt, Gross, Current Maturities		688		0
Debt Instrument, Unamortized Premium (Discount) and debt issuance cost, net, Current Maturities		22		0
Long-term Debt, Current Maturities		710		0
Long-term Debt, Gross Excluding Current Maturities		3,165		3,678
Debt Instrument, Unamortized Premium (Discount) and Debt Issuance Cost, Net, Excluding Current Maturities		(38)		1
Long-term Debt, Excluding Current Maturities		3,127		3,679
Senior Secured Revolving Credit Facility [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Principal amount		0		325
Unamortized Premium (Discount) and Debt Issuance Costs		0		0
Long-term debt, net		$ 0		325
Senior Notes [Member] \| 4.625% Westlake Senior Notes 2021 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2021
Stated interest rate (percent)		4.625%
Principal amount	$ 625	$ 625		625
Unamortized Premium (Discount) and Debt Issuance Costs		20		27
Long-term debt, net		$ 645		652
Senior Notes [Member] \| 4.625% Subsidiary Senior Notes 2021 [Member]
Debt Instrument [Line Items]
Debt instrument maturity	2021	2021
Stated interest rate (percent)	4.625%	4.625%
Principal amount	$ 688	$ 63		63
Unamortized Premium (Discount) and Debt Issuance Costs		2		3
Long-term debt, net		$ 65		66
Senior Notes [Member] \| 3.60% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2022
Stated interest rate (percent)		3.60%
Principal amount		$ 250		250		$ 250
Unamortized Premium (Discount) and Debt Issuance Costs		(1)		(2)
Long-term debt, net		$ 249		248
Senior Notes [Member] \| 4.875% Westlake Senior 2023 Notes [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2023
Stated interest rate (percent)		4.875%
Principal amount	$ 434	$ 434		434
Unamortized Premium (Discount) and Debt Issuance Costs		11		13
Long-term debt, net		$ 445		447
Senior Notes [Member] \| 4.875% Subsidiary 2023 Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument maturity	2023	2023
Stated interest rate (percent)	4.875%	4.875%
Principal amount	$ 450	$ 16		16
Unamortized Premium (Discount) and Debt Issuance Costs		0		1
Long-term debt, net		$ 16		17
Senior Notes [Member] \| 3.60% Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2026
Stated interest rate (percent)		3.60%
Principal amount		$ 750		750	$ 750
Unamortized Premium (Discount) and Debt Issuance Costs		(10)		(11)
Long-term debt, net		$ 740		739
Senior Notes [Member] \| Waste Disposal Revenue Bonds Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2027
Principal amount		$ 11		11
Unamortized Premium (Discount) and Debt Issuance Costs		0		0
Long-term debt, net		$ 11		11
Senior Notes [Member] \| 6 1/2% GO Zone Senior Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2029
Stated interest rate (percent)		6.50%
Principal amount		$ 100		100
Unamortized Premium (Discount) and Debt Issuance Costs		(1)		(1)
Long-term debt, net		$ 99		99
Senior Notes [Member] \| 6.75% Go Zone Senior Notes Due 2032 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2032
Stated interest rate (percent)		6.75%
Principal amount		$ 0		250
Unamortized Premium (Discount) and Debt Issuance Costs		0		(2)
Long-term debt, net		$ 0		248
Senior Notes [Member] \| 6 1/2% GO Zone Tax-Exempt Revenue Bonds Due 2035 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2035
Stated interest rate (percent)		6.50%
Principal amount		$ 89		89			$ 89
Unamortized Premium (Discount) and Debt Issuance Costs		(1)		(1)
Long-term debt, net		$ 88		88
Senior Notes [Member] \| 6.5% 2035 IKE Zone Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2035
Stated interest rate (percent)		6.50%
Principal amount		$ 65		65			$ 65
Unamortized Premium (Discount) and Debt Issuance Costs		0		0
Long-term debt, net		$ 65		65
Senior Notes [Member] \| 5.0% 2046 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2046
Stated interest rate (percent)		5.00%
Principal amount		$ 700		700
Unamortized Premium (Discount) and Debt Issuance Costs		(25)		(26)
Long-term debt, net		$ 675		674
Senior Notes [Member] \| 4.375% Senior Notes Due 2047 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2047
Stated interest rate (percent)		4.375%
Principal amount		$ 500	$ 500	0
Unamortized Premium (Discount) and Debt Issuance Costs		(9)		0
Long-term debt, net		$ 491		0
Senior Notes [Member] \| 3.50% Senior Notes Due 2032 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2032
Stated interest rate (percent)		3.50%
Principal amount		$ 250		0
Unamortized Premium (Discount) and Debt Issuance Costs		(2)		0
Long-term debt, net		$ 248		$ 0