Quarterly Financial Data (Schedule Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	[1]
Revenues	$ 28,250	$ 26,449	$ 25,065	$ 27,229	$ 28,285	$ 26,205	$ 25,493	$ 23,789			$ 106,993	$ 103,772	$ 50,937
Operating expenses	10,635	10,548	10,655	10,997	10,693	11,474	11,043	11,071			42,835	44,281	38,543
Operating income	17,615	15,901	14,410	16,232	17,592	14,731	14,450	12,718			64,158	59,491	12,394
Other income (expense)	(1,291)	(1,392)	(1,280)	(1,208)	(1,230)	(480)	(389)	(363)			(5,171)	(2,462)	(295)
Income tax (expense) benefit	26	$ (43)	$ (45)	$ (47)	80	(52)	(79)	(173)			(109)	(224)	4,009
Equity investment loss	$ (11)										(11)
Net income (loss)					$ 16,442	$ 14,199	$ 13,982	$ 12,182			$ 58,867	$ 56,805	$ 23,009
Distribution declared	$ 0.4700	$ 0.4600	$ 0.4500	$ 0.4400	$ 0.4300	$ 0.4200	$ 0.4100	$ 0.4050	$ 0.4025	$ 0.4000
Common Units [Member]
Earnings per limited partner unit (basic and diluted)	0.50	0.45	0.40	0.46	0.50	0.44	0.43	0.38
Subordinated Units [Member]
Earnings per limited partner unit (basic and diluted)	$ 0.50	$ 0.45	$ 0.40	$ 0.46	$ 0.50	$ 0.44	$ 0.43	$ 0.38

[1] Recast to include historical results of operations related to net assets acquired in a transfer between entities under common control. See Notes 1 and 4 in the accompanying notes to consolidated financial statements for further discussion.