Related Party Transactions (General) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Buyer deficiency fee	$ 5,500		$ 69,555
Shortfall	4,178
Investments with Westlake under the Investment Management Agreement	$ 64,000	$ 78,000
IMA, Partnership earned amount added to market rate, basis points	five
Affiliated Entity | Westlake
Related Party Transaction [Line Items]
Interest earned related to the IMA		587
Operating Lease, Expense	$ 686	$ 670
Basis points above market rate for IMA	67
Accrued Liabilities | Affiliated Entity | Westlake
Related Party Transaction [Line Items]
Accrued interest included in receivable under IMA	$ 67		$ 56