Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Debt Instrument, Maturity

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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, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt		$ 2,668	$ 3,837
Senior Notes [Member] \| 3.60% Senior Notes Due 2022 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt		$ 249	249
Debt instrument maturity		2022
Stated interest rate (percent)		3.60%
Senior Notes [Member] \| 3.60% Senior Notes Due 2026 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt		$ 741	740
Debt instrument maturity		2026
Stated interest rate (percent)		3.60%
Senior Notes [Member] \| Loan related to tax-exempt waste disposal revenue bonds due 2027 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt		$ 11	11
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]
Long-term Debt		$ 99	99
Debt instrument maturity		2029
Stated interest rate (percent)		6.50%
Senior Notes [Member] \| 6 1/2% GO Zone Senior Notes Due 2035 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt		$ 88	88
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]
Long-term Debt		$ 65	65
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]
Long-term Debt		$ 676	675
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]
Long-term Debt		$ 491	491
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]
Long-term Debt		$ 248	248
Debt instrument maturity		2032
Stated interest rate (percent)		3.50%
Senior Notes [Member] \| 4.625% Westlake Senior Notes 2021 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt		$ 0	645
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]
Long-term Debt		$ 0	65
Debt instrument maturity	2021	2021
Stated interest rate (percent)	4.625%	4.625%
Senior Notes [Member] \| 4.875% Westlake Senior 2023 Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt		$ 0	445
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]
Long-term Debt		$ 0	16
Debt instrument maturity	2023	2023
Stated interest rate (percent)	4.875%	4.875%
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		$ 248	255
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		692	757
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		106	111
Estimate of Fair Value Measurement [Member] \| Senior Notes [Member] \| 6 1/2% GO Zone Senior Notes Due 2035 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		95	99
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		69	74
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		641	787
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		417	518
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		233	256
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		0	639
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		0	65
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		0	449
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		0	16
Estimate of Fair Value Measurement [Member] \| Loan related to tax-exempt waste disposal revenue bonds due 2027 [Member] \| Loan related to tax-exempt waste disposal revenue bonds due 2027 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes		$ 11	$ 11