Partners' Capital, Mezzanine Equity and Distributions (Distributions Paid) (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 14, 2019	Nov. 14, 2018	Aug. 14, 2018	May 15, 2018	Feb. 14, 2018	Nov. 14, 2017	Aug. 14, 2017	May 15, 2017	Feb. 14, 2017	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]
Distribution Made to Limited Partner [Line Items]
Date Paid							Aug. 14, 2017	May 15, 2017	Feb. 14, 2017
Per Unit Amount (in dollars per unit)		$ 0.5400	$ 0.5300	$ 0.5200	$ 0.5100	$ 0.5000	$ 0.7225	$ 0.7200	$ 0.7100	$ 0.5500		$ 0.5400		$ 0.5300		$ 0.5200		$ 0.5000		$ 0.7225		$ 0.7200		$ 0.7100
Total Amount		$ 66,193	$ 64,967	$ 63,741	$ 62,515	$ 61,290	$ 88,563	$ 88,257	$ 83,765
Date Issued		Nov. 14, 2018	Aug. 14, 2018	May 15, 2018	Feb. 14, 2018	Nov. 14, 2017
Scenario, Forecast | Subsequent Event
Distribution Made to Limited Partner [Line Items]
Date Paid	Feb. 14, 2019
Per Unit Amount (in dollars per unit)	$ 0.5500
Total Amount	$ 67,419

[1] (1)Represents cash distributions declared and paid in the applicable period.