Related Party Transactions (Debt Payable to Related Parties) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Interest on related party debt payable	$ 5,900	$ 4,866
Accrued interest on related party debt	5,852		$ 5,448
Long-term debt payable to Westlake	601,119		477,608
Affiliated Entity [Member] \| Westlake [Member]
Related Party Transaction [Line Items]
Interest capitalized	0	15
Other income (expense) [Member] \| Affiliated Entity [Member] \| Westlake [Member]
Related Party Transaction [Line Items]
Interest on related party debt payable	5,900	$ 4,866
Accrued Liabilities [Member] \| Affiliated Entity [Member] \| Westlake [Member]
Related Party Transaction [Line Items]
Accrued interest on related party debt	$ 5,852		$ 5,448

Portion of the carrying amount as of the balance sheet date of obligations due all related parties that is payable after one year or beyond the normal operating cycle if longer.

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Amount of interest capitalized during the period.

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