Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands																12 Months Ended
		Feb. 12, 2016	Feb. 01, 2016	Nov. 13, 2015	Aug. 14, 2015	May. 15, 2015	Feb. 13, 2015	Nov. 14, 2014	Aug. 14, 2014	May. 15, 2014	Feb. 14, 2014	Nov. 14, 2013	Aug. 14, 2013	May. 15, 2013	Feb. 14, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Subsequent Event [Line Items]
Cash distribution per limited partner unit				$ 0.76250	$ 0.74000	$ 0.71750	$ 0.69500	$ 0.66750	$ 0.64000	$ 0.61250	$ 0.58500	$ 0.55750	$ 0.53250	$ 0.50750	$ 0.50	$ 2.915	$ 2.505	$ 2.0975
Total cash distributions				$ 173,413	$ 168,296	$ 163,178	$ 158,061	$ 151,568	$ 145,324	$ 139,079	$ 132,835	$ 126,374	$ 120,707	$ 115,040	$ 113,340	$ 662,948	$ 568,806	$ 475,461
Subsequent Event [Member]
Subsequent Event [Line Items]
Units granted			218,046
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			3 years
Partners' Capital Account, Units, Unit-based Compensation			353,786
Cash distribution per limited partner unit		$ 0.785
Total cash distributions		$ 178,800
Management [Member]
Subsequent Event [Line Items]
Units granted																174,449
Partners' Capital Account, Units, Unit-based Compensation																354,529	387,216	476,682
Management [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation			350,552
Director [Member]
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation	[1]															4,461	1,603	1,884
Director [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation			3,234

[1] Limited partner units issued to settle the equity-based retainer paid to independent directors of our general partner.