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Fair Value Measurements (Summary of Carrying and Fair Values of Long Term Debt) (Details) - USD ($) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016
Senior Notes [Member] | 4.625% Senior Notes Due 2021 (New Westlake 2021 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Debt Instrument Maturity | 2021 |  |
Stated interest rate (percent) | 4.625% |  | 4.625%
Senior Notes [Member] | 4.625% Senior Notes Due 2021 (Existing Axiall 2021 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Debt Instrument Maturity | 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% Senior Notes Due 2023 (New Westlake 2023 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Debt Instrument Maturity | 2023 |  |
Stated interest rate (percent) | 4.875% |  | 4.875%
Senior Notes [Member] | 4.875% Senior Notes Due 2023 (Existing Axiall 2023 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Debt Instrument Maturity | 2023 |  |
Stated interest rate (percent) | 4.875% |  | 4.875%
Senior Notes [Member] | 3.6% 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] | Loan related to tax-exempt 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% 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 3/4% 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] | 2035 GO Zone 6 1/2% 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] | 2035 IKE Zone 6 1/2% 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% Senior Notes Due 2046 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Debt Instrument Maturity | 2046 |  |
Stated interest rate (percent) | 5.00% |  |
Carrying Value [Member] | Line of Credit [Member] | Unsecured Revolving Credit Facility [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Revolving credit facility | $ 250,000 | $ 325,000 |
Carrying Value [Member] | Senior Notes [Member] | 4.625% Senior Notes Due 2021 (New Westlake 2021 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 624,793 | 651,630 |
Carrying Value [Member] | Senior Notes [Member] | 4.625% Senior Notes Due 2021 (Existing Axiall 2021 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 63,207 | 66,069 |
Carrying Value [Member] | Senior Notes [Member] | 3.60% Senior Notes Due 2022 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 250,000 | 248,109 |
Carrying Value [Member] | Senior Notes [Member] | 4.875% Senior Notes Due 2023 (New Westlake 2023 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 433,793 | 447,224 |
Carrying Value [Member] | Senior Notes [Member] | 4.875% Senior Notes Due 2023 (Existing Axiall 2023 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 16,207 | 16,747 |
Carrying Value [Member] | Senior Notes [Member] | 3.6% Senior Notes Due 2026 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 750,000 | 739,243 |
Carrying Value [Member] | Senior Notes [Member] | 6 1/2% Senior Notes Due 2029 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 100,000 | 99,084 |
Carrying Value [Member] | Senior Notes [Member] | 6 3/4% Senior Notes Due 2032 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 250,000 | 248,117 |
Carrying Value [Member] | Senior Notes [Member] | 2035 GO Zone 6 1/2% Notes [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 89,000 | 88,161 |
Carrying Value [Member] | Senior Notes [Member] | 2035 IKE Zone 6 1/2% Notes [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 65,000 | 64,398 |
Carrying Value [Member] | Senior Notes [Member] | 5% Senior Notes Due 2046 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 700,000 | 673,983 |
Carrying Value [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 | 10,889 | 10,889 |
Fair Value [Member] | Line of Credit [Member] | Unsecured Revolving Credit Facility [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Revolving credit facility | 250,000 | 325,000 |
Fair Value [Member] | Senior Notes [Member] | 4.625% Senior Notes Due 2021 (New Westlake 2021 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 649,354 | 650,847 |
Fair Value [Member] | Senior Notes [Member] | 4.625% Senior Notes Due 2021 (Existing Axiall 2021 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 65,639 | 65,775 |
Fair Value [Member] | Senior Notes [Member] | 3.60% Senior Notes Due 2022 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 254,443 | 251,725 |
Fair Value [Member] | Senior Notes [Member] | 4.875% Senior Notes Due 2023 (New Westlake 2023 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 451,761 | 451,301 |
Fair Value [Member] | Senior Notes [Member] | 4.875% Senior Notes Due 2023 (Existing Axiall 2023 Notes) [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 16,873 | 16,501 |
Fair Value [Member] | Senior Notes [Member] | 3.6% Senior Notes Due 2026 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 735,870 | 722,055 |
Fair Value [Member] | Senior Notes [Member] | 6 1/2% Senior Notes Due 2029 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 110,000 | 112,433 |
Fair Value [Member] | Senior Notes [Member] | 6 3/4% Senior Notes Due 2032 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 258,760 | 258,818 |
Fair Value [Member] | Senior Notes [Member] | 2035 GO Zone 6 1/2% Notes [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 101,760 | 100,323 |
Fair Value [Member] | Senior Notes [Member] | 2035 IKE Zone 6 1/2% Notes [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 74,298 | 73,270 |
Fair Value [Member] | Senior Notes [Member] | 5% Senior Notes Due 2046 [Member] |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Senior notes | 717,864 | 691,712 |
Fair Value [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 | $ 10,889 | $ 10,889 |

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