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Equity Cash distributions (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Jan. 24, 2017
Nov. 14, 2016
Aug. 12, 2016
May 13, 2016
Feb. 12, 2016
Nov. 13, 2015
Aug. 14, 2015
May 14, 2015
Feb. 13, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Distribution Made to Limited Partner [Line Items]                        
Quarterly cash distribution (dollars per unit)   $ 0.655 $ 0.630 $ 0.610 $ 0.590 $ 0.570 $ 0.550 $ 0.530 $ 0.510      
Cash distributions per limited partner unit                   $ 2.575 $ 2.240 $ 1.900
Distribution Made To Member Or Limited Partner Distributions Paid Annualized Per Unit   $ 2.62 $ 2.52 $ 2.44 $ 2.36 $ 2.28 $ 2.20 $ 2.12 $ 2.04      
Cash Distributions Paid   $ 19,302 $ 18,085 $ 17,095 $ 16,124 $ 15,136 $ 14,368 $ 13,702 $ 13,056      
General partner's distributions                   $ 1,277 $ 1,109 $ 938
General partner's IDRs                   11,160 3,904 945
Total general partner's distributions                   12,437 5,013 1,883
Limited partners' distribution                   62,582 54,317 45,951
Total cash distributions                   75,019 59,330 47,834
Subsequent Event [Member]                        
Distribution Made to Limited Partner [Line Items]                        
Cash distributions per limited partner unit $ 0.680                      
Distribution Made To Limited Partner Distributions Declared Annualized Per Unit $ 2.72                      
Cash Distributions Declared $ 20,537                      
Limited Partner [Member] | Common                        
Distribution Made to Limited Partner [Line Items]                        
Limited partners' distribution                   58,158 27,439 23,152
Limited Partner [Member] | Subordinated                        
Distribution Made to Limited Partner [Line Items]                        
Limited partners' distribution                   $ 4,424 $ 26,878 $ 22,799