Equity Cash distributions (Details) - USD ($) $ / shares in Units, $ in Thousands										12 Months Ended
	Jan. 25, 2016	Nov. 13, 2015	Aug. 14, 2015	May. 14, 2015	Feb. 13, 2015	Nov. 14, 2014	Aug. 14, 2014	May. 14, 2014	Feb. 13, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Distribution Made to Limited Partner [Line Items]
Quarterly cash distribution (dollars per unit)		$ 0.570	$ 0.550	$ 0.530	$ 0.510	$ 0.490	$ 0.475	$ 0.425	$ 0.415
Distribution Made To Member Or Limited Partner Distributions Paid Annualized Per Unit		$ 2.28	$ 2.20	$ 2.12	$ 2.04	$ 1.96	$ 1.90	$ 1.70	$ 1.66
Cash Distributions Paid		$ 15,136	$ 14,368	$ 13,702	$ 13,056	$ 12,394	$ 11,910	$ 10,474	$ 10,228
General partner's distributions										$ 1,109	$ 938	$ 785
General partner's IDRs										3,904	945	0
Total general partner's distributions										5,013	1,883	785
Limited partners' distribution										54,317	45,951	38,491
Total cash distributions										$ 59,330	$ 47,834	$ 39,276
Cash distributions per limited partner unit										$ 2.240	$ 1.900	$ 1.600
Subsequent Event [Member]
Distribution Made to Limited Partner [Line Items]
Cash Distributions Declared	$ 16,124
Cash distributions per limited partner unit	$ 0.590
Distribution Made To Limited Partner Distributions Declared Annualized Per Unit	$ 2.36
Limited Partner [Member] | Common
Distribution Made to Limited Partner [Line Items]
Limited partners' distribution										$ 27,439	$ 23,152	$ 19,292
Limited Partner [Member] | Subordinated
Distribution Made to Limited Partner [Line Items]
Limited partners' distribution										$ 26,878	$ 22,799	$ 19,199