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Equity Cash distributions (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended
Jul. 24, 2017
May 12, 2017
Feb. 14, 2017
Nov. 14, 2016
Aug. 12, 2016
May 13, 2016
Feb. 12, 2016
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Distribution Made to Limited Partner [Line Items]                      
Distribution Made to Limited Partner, Distributions Paid, Per Unit   $ 0.690 $ 0.680 $ 0.655 $ 0.630            
Distribution Made To Member Or Limited Partner Distributions Paid Annualized Per Unit   $ 2.76 $ 2.72 $ 2.62 $ 2.52            
Quarterly cash distributions paid   $ 21,024 $ 20,537 $ 19,302 $ 18,085 $ 17,100 $ 16,100        
Distribution Made to Limited Partner, Distributions Declared, Per Unit               $ 0.705 $ 0.630 $ 1.395 $ 1.240
Subsequent Event [Member]                      
Distribution Made to Limited Partner [Line Items]                      
Distribution Made To Member Or Limited Partner Distributions Paid Annualized Per Unit $ 2.82                    
Distribution Made to Limited Partner, Distributions Declared, Per Unit $ 0.705                    
Distribution Made to Limited Partner, Cash Distributions Declared $ 21,783