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Long-Term Debt (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016
Debt Instrument [Line Items] |  |  |
Principal Amount | $ 3,602,889 | $ 3,677,889 |
Unamortized Premium, Discount and Debt Issuance Costs, | (1,247) | 765 |
Net, Long-term Debt | 3,601,642 | 3,678,654 |
Line of Credit [Member] | Revolving Credit Facility [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Principal Amount | 250,000 | 325,000 |
Unamortized Premium, Discount and Debt Issuance Costs, | 0 | 0 |
Net, Long-term Debt | $ 250,000 | 325,000 |
Senior Notes [Member] | 4.625% Senior Notes Due 2021 (New Westlake 2021 Notes) [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2021 |  |
Stated interest rate (percent) | 4.625% |  | 4.625%
Principal Amount | $ 624,793 | 624,793 |
Unamortized Premium, Discount and Debt Issuance Costs, | 25,123 | 26,837 |
Net, Long-term Debt | $ 649,916 | 651,630 |
Senior Notes [Member] | 4.625% Senior Notes Due 2021 (Existing Axiall 2021 Notes) [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2021 |  |
Stated interest rate (percent) | 4.625% |  | 4.625%
Principal Amount | $ 63,207 | 63,207 | $ 688,000
Unamortized Premium, Discount and Debt Issuance Costs, | 2,690 | 2,862 |
Net, Long-term Debt | $ 65,897 | 66,069 |
Senior Notes [Member] | 3.60% Senior Notes Due 2022 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2022 |  |
Stated interest rate (percent) | 3.60% |  |
Principal Amount | $ 250,000 | 250,000 |
Unamortized Premium, Discount and Debt Issuance Costs, | (1,806) | (1,891) |
Net, Long-term Debt | $ 248,194 | 248,109 |
Senior Notes [Member] | 4.875% Senior Notes Due 2023 (New Westlake 2023 Notes) [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2023 |  |
Stated interest rate (percent) | 4.875% |  | 4.875%
Principal Amount | $ 433,793 | 433,793 |
Unamortized Premium, Discount and Debt Issuance Costs, | 12,840 | 13,431 |
Net, Long-term Debt | $ 446,633 | 447,224 |
Senior Notes [Member] | 4.875% Senior Notes Due 2023 (Existing Axiall 2023 Notes) [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2023 |  |
Stated interest rate (percent) | 4.875% |  | 4.875%
Principal Amount | $ 16,207 | 16,207 | $ 450,000
Unamortized Premium, Discount and Debt Issuance Costs, | 519 | 540 |
Net, Long-term Debt | $ 16,726 | 16,747 |
Senior Notes [Member] | 3.6% Senior Notes Due 2026 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2026 |  |
Stated interest rate (percent) | 3.60% |  |
Principal Amount | $ 750,000 | 750,000 |
Unamortized Premium, Discount and Debt Issuance Costs, | (10,564) | (10,757) |
Net, Long-term Debt | $ 739,436 | 739,243 |
Senior Notes [Member] | Loan related to tax-exempt waste disposal revenue bonds due 2027 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2027 |  |
Principal Amount | $ 10,889 | 10,889 |
Unamortized Premium, Discount and Debt Issuance Costs, | 0 | 0 |
Net, Long-term Debt | $ 10,889 | 10,889 |
Senior Notes [Member] | 6 1/2% Senior Notes Due 2029 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2029 |  |
Stated interest rate (percent) | 6.50% |  |
Principal Amount | $ 100,000 | 100,000 |
Unamortized Premium, Discount and Debt Issuance Costs, | (898) | (916) |
Net, Long-term Debt | $ 99,102 | 99,084 |
Senior Notes [Member] | 6 3/4% Senior Notes Due 2032 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2032 |  |
Stated interest rate (percent) | 6.75% |  |
Principal Amount | $ 250,000 | 250,000 |
Unamortized Premium, Discount and Debt Issuance Costs, | (1,853) | (1,883) |
Net, Long-term Debt | $ 248,147 | 248,117 |
Senior Notes [Member] | 2035 GO Zone 6 1/2% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2035 |  |
Stated interest rate (percent) | 6.50% |  |
Principal Amount | $ 89,000 | 89,000 |
Unamortized Premium, Discount and Debt Issuance Costs, | (828) | (839) |
Net, Long-term Debt | $ 88,172 | 88,161 |
Senior Notes [Member] | 2035 IKE Zone 6 1/2% Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2035 |  |
Stated interest rate (percent) | 6.50% |  |
Principal Amount | $ 65,000 | 65,000 |
Unamortized Premium, Discount and Debt Issuance Costs, | (594) | (602) |
Net, Long-term Debt | $ 64,406 | 64,398 |
Senior Notes [Member] | 5% Senior Notes Due 2046 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity | 2046 |  |
Stated interest rate (percent) | 5.00% |  |
Principal Amount | $ 700,000 | 700,000 |
Unamortized Premium, Discount and Debt Issuance Costs, | (25,876) | (26,017) |
Net, Long-term Debt | $ 674,124 | $ 673,983 |

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