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Guarantor Disclosures (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Condensed Financial Statements, Captions [Line Items]  
Ownership interest (in percent) 100.00%
Senior Notes [Member] | 4.375% Senior Notes Due 2047 [Member]  
Condensed Financial Statements, Captions [Line Items]  
Stated interest rate (percent) 4.375%
Debt instrument maturity 2047
Senior Notes [Member] | 3.60% Senior Notes Due 2022 [Member]  
Condensed Financial Statements, Captions [Line Items]  
Stated interest rate (percent) 3.60%
Debt instrument maturity 2022
Senior Notes [Member] | 3.60% Senior Notes Due 2026 [Member]  
Condensed Financial Statements, Captions [Line Items]  
Stated interest rate (percent) 3.60%
Debt instrument maturity 2026
Senior Notes [Member] | 5.0% 2046 [Member]  
Condensed Financial Statements, Captions [Line Items]  
Stated interest rate (percent) 5.00%
Debt instrument maturity 2046
Senior Notes [Member] | 4.875% Westlake Senior 2023 Notes [Member]  
Condensed Financial Statements, Captions [Line Items]  
Stated interest rate (percent) 4.875%
Debt instrument maturity 2023
3.60%, 5.0%, 4.625%, 4.875% Senior Notes [Member]  
Condensed Financial Statements, Captions [Line Items]  
Senior notes, minimum debt amount guaranteed by subsidiaries $ 5