Business Acquisitions (Details 4) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 17, 2015	Aug. 08, 2014	Apr. 10, 2014	Apr. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Business Acquisitions
Payment for business acquisition								$ 351,121
Pro forma financial information
Revenue					$ 169,113	$ 159,840	$ 341,175	313,941
Net income					$ 22,488	$ 19,188	$ 43,004	$ 28,948
Basic earnings per common unit (in dollars per unit)					$ 0.30	$ 0.27	$ 0.56	$ 0.38
Diluted earnings per common unit (in dollars per unit)					$ 0.30	$ 0.27	$ 0.56	$ 0.38
Additional pro forma reduction of net income allocable to the entity's limited partners						$ 200	$ 300	$ 400
Revolving credit facility due May 2018
Business Acquisitions
Payments on revolving credit facility				$ 157,500
6% senior notes due October 2022
Business Acquisitions
Principle amount of senior notes				$ 350,000
Partners' Capital Common Units
Business Acquisitions
Units sold (in units)				6,210,000			49,774	6,210,000
Units issued in connection with acquisition (in units)							3,963,138
Partners' Capital General Partner Units
Business Acquisitions
Issuance of general partner units to general partner				126,000				125,994
Units issued in connection with acquisition (in units)							80,341
April 2015 Contract Operations Acquisition | Partners' Capital Common Units
Business Acquisitions
Units issued in connection with acquisition (in units)	4,000,000
April 2015 Contract Operations Acquisition | Partners' Capital General Partner Units
Business Acquisitions
Units issued in connection with acquisition (in units)	80,000
August 2014 MidCon Acquisition
Business Acquisitions
Payment for business acquisition		$ 130,100
April 2014 MidCon Acquisition
Business Acquisitions
Payment for business acquisition			$ 352,900