Fair Value Measurements (Schedule Of Carrying And Fair Values Of Long-Term Debt) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
Senior Notes [Member] | 4.625% Westlake Senior Notes 2021 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2021
Stated interest rate (percent)		4.625%
Senior Notes [Member] | 4.625% Subsidiary Senior Notes 2021 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity	2021	2021
Stated interest rate (percent)	4.625%	4.625%
Senior Notes [Member] | 3.60% Senior Notes Due 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2022
Stated interest rate (percent)		3.60%
Senior Notes [Member] | 4.875% Westlake Senior 2023 Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2023
Stated interest rate (percent)		4.875%
Senior Notes [Member] | 4.875% Subsidiary 2023 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity	2023	2023
Stated interest rate (percent)	4.875%	4.875%
Senior Notes [Member] | 3.60% Senior Notes Due 2026 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2026
Stated interest rate (percent)		3.60%
Senior Notes [Member] | Waste Disposal Revenue Bonds Due 2027 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2027
Senior Notes [Member] | 6 1/2% GO Zone Senior Notes Due 2029 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2029
Stated interest rate (percent)		6.50%
Senior Notes [Member] | 6.75% Go Zone Senior Notes Due 2032 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2032
Stated interest rate (percent)		6.75%
Senior Notes [Member] | 6.5% 2035 GO Zone Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2035
Stated interest rate (percent)		6.50%
Senior Notes [Member] | 6.5% 2035 IKE Zone Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2035
Stated interest rate (percent)		6.50%
Senior Notes [Member] | 5.0% 2046 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2046
Stated interest rate (percent)		5.00%
Senior Notes [Member] | 4.375% Senior Notes Due 2047 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2047
Stated interest rate (percent)		4.375%
Senior Notes [Member] | 3.50% Senior Notes Due 2032 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument maturity		2032
Stated interest rate (percent)		3.50%
Reported Value Measurement [Member] | Senior Notes [Member] | 4.625% Westlake Senior Notes 2021 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		$ 645	$ 652
Reported Value Measurement [Member] | Senior Notes [Member] | 4.625% Subsidiary Senior Notes 2021 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		65	66
Reported Value Measurement [Member] | Senior Notes [Member] | 3.60% Senior Notes Due 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		249	248
Reported Value Measurement [Member] | Senior Notes [Member] | 4.875% Westlake Senior 2023 Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		445	447
Reported Value Measurement [Member] | Senior Notes [Member] | 4.875% Subsidiary 2023 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		16	17
Reported Value Measurement [Member] | Senior Notes [Member] | 3.60% Senior Notes Due 2026 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		740	739
Reported Value Measurement [Member] | Senior Notes [Member] | 6 1/2% GO Zone Senior Notes Due 2029 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		99	99
Reported Value Measurement [Member] | Senior Notes [Member] | 6.75% Go Zone Senior Notes Due 2032 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		0	248
Reported Value Measurement [Member] | Senior Notes [Member] | 6.5% 2035 GO Zone Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		88	88
Reported Value Measurement [Member] | Senior Notes [Member] | 6.5% 2035 IKE Zone Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		65	65
Reported Value Measurement [Member] | Senior Notes [Member] | 5.0% 2046 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		675	674
Reported Value Measurement [Member] | Senior Notes [Member] | 4.375% Senior Notes Due 2047 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		491	0
Reported Value Measurement [Member] | Senior Notes [Member] | 3.50% Senior Notes Due 2032 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		248	0
Reported Value Measurement [Member] | Waste Disposal Revenue Bonds Due 2027 [Member] | Waste Disposal Revenue Bonds Due 2027 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		11	11
Reported Value Measurement [Member] | Revolving Credit Facility [Member] | Line of Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Revolving credit facility		0	325
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 4.625% Westlake Senior Notes 2021 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		639	651
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 4.625% Subsidiary Senior Notes 2021 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		65	66
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 3.60% Senior Notes Due 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		255	252
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 4.875% Westlake Senior 2023 Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		449	451
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 4.875% Subsidiary 2023 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		16	17
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 3.60% Senior Notes Due 2026 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		757	722
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 6 1/2% GO Zone Senior Notes Due 2029 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		111	112
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 6.75% Go Zone Senior Notes Due 2032 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		0	259
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 6.5% 2035 GO Zone Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		99	100
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 6.5% 2035 IKE Zone Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		74	73
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 5.0% 2046 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		787	692
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 4.375% Senior Notes Due 2047 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		518	0
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | 3.50% Senior Notes Due 2032 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		256	0
Estimate of Fair Value Measurement [Member] | Waste Disposal Revenue Bonds Due 2027 [Member] | Waste Disposal Revenue Bonds Due 2027 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		11	11
Estimate of Fair Value Measurement [Member] | Revolving Credit Facility [Member] | Line of Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Revolving credit facility		$ 0	$ 325