Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Contractual interest rate for funds borrowed, under the debt agreement.

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Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

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Exchange Offer Percentage

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Long-Term Debt (Senior Notes Due 2021 and 2023) (Details) - USD ($) $ in Millions				1 Months Ended	12 Months Ended
	May 15, 2018	Feb. 15, 2018	Apr. 24, 2017	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 2,715	$ 3,853
Debt Instrument, Redemption Price, Percentage		102.313%
2023 Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage	102.438%
Senior Notes [Member] \| 4.625% Westlake Senior Notes 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 625	$ 0	625
Stated interest rate (percent)					4.625%
Debt instrument maturity					2021
Exchange Offer Percentage			100.00%
Senior Notes [Member] \| 4.875% Westlake Senior 2023 Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				434	$ 0	434
Stated interest rate (percent)					4.875%
Debt instrument maturity					2023
Senior Notes [Member] \| 4.625% Subsidiary Senior Notes 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 688	$ 0	63
Stated interest rate (percent)				4.625%	4.625%
Debt instrument maturity				2021	2021
Senior Notes [Member] \| 4.875% Subsidiary 2023 Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 450	$ 0	$ 16
Stated interest rate (percent)				4.875%	4.875%
Debt instrument maturity				2023	2023