Distributions and Net Income Per Limited Partner Unit (Narrative) (Details) - $ / shares		12 Months Ended
	Jan. 24, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 30, 2017
Common and Subordinated units [Member] | Cash Distribution [Member]
Distributions [Line Items]
Minimum quarterly distribution per unit (usd per share)		$ 1.2938
Common and Subordinated units [Member] | Cash Distribution [Member] | Subsequent Event [Member]
Distributions [Line Items]
Distribution declared per unit (usd per share)	$ 0.4714
Westlake [Member] | Common units [Member]
Distributions [Line Items]
Units outstanding		14,122,230	14,122,230
Westlake [Member] | Subordinated units [Member]
Distributions [Line Items]
Units outstanding				12,686,115