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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Long-Term Debt (Senior Notes Due 2026 and 2046) (Details) - USD ($) $ in Millions	Apr. 24, 2017	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Aug. 10, 2016
Debt Instrument [Line Items]
Principal amount		$ 3,853	$ 3,678
3.60% Senior Notes Due 2026 [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Principal amount		750	750		$ 750
Exchange Offer Percentage	99.97%
5.0% Senior Notes Due 2046 [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Principal amount					$ 700
4.875% Westlake Senior 2023 Notes [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Principal amount		$ 434	$ 434	$ 434
Exchange Offer Percentage	100.00%