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Summary of Quarterly Financial Information (unaudited) (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
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]                              
Revenues $ 183,070 $ 187,320 $ 177,124 $ 164,954 $ 179,263 $ 175,848 $ 165,815 $ 157,074 $ 165,200 $ 175,779 $ 164,773 $ 151,882 $ 712,468 $ 678,000 $ 657,634
Operating income 50,964 56,781 51,434 36,666 52,651 53,586 44,192 34,114 37,230 45,343 45,039 23,757 195,845 184,543 151,369
Net income 32,260 35,489 31,359 19,145 33,008 32,897 24,640 16,705 19,370 26,085 26,585 7,888 118,253 107,250 79,928
Net income attributable to common unitholders $ 32,026 $ 35,255 $ 31,125 $ 18,911 $ 32,774 $ 32,663 $ 24,406 $ 16,471 $ 19,136 $ 25,851 $ 26,351 $ 7,654 $ 117,317 $ 106,314 $ 78,992
Basic and diluted net income per share (in dollars per share) $ 0.11 $ 0.12 $ 0.10 $ 0.06 $ 0.11 $ 0.12 $ 0.09 $ 0.06 $ 0.07 $ 0.10 $ 0.10 $ 0.03 $ 0.39 $ 0.38 $ 0.29