Related Party Transactions Related Party Transactions (Receivable under the Investment Management Agreement) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Receivable under the Investment Management Agreement—Westlake Chemical Corporation (Westlake)	$ 162,773	$ 148,956
Westlake [Member] | Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest earned related to the IMA	3,289	2,646	$ 340
Westlake [Member] | Accrued Liabilities [Member] | Affiliated Entity [Member]
Related Party Transaction [Line Items]
Accrued interest included in receivable under IMA	$ 601	$ 496