Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands																	12 Months Ended
		Feb. 14, 2018	Feb. 01, 2018	Jan. 31, 2018	Nov. 14, 2017	Aug. 14, 2017	May 15, 2017	Feb. 14, 2017	Nov. 14, 2016	Aug. 12, 2016	May 13, 2016	Feb. 12, 2016	Nov. 13, 2015	Aug. 14, 2015	May 15, 2015	Feb. 13, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation																	216,679	350,552	354,529
Units granted																	220,148
Partners' Capital Account, Distribution Per Unit of Limited Partner Interest					$ 0.9050	$ 0.8900	$ 0.8725	$ 0.8550	$ 0.83750	$ 0.8200	$ 0.8025	$ 0.7850	$ 0.7625	$ 0.7400	$ 0.7175	$ 0.6950	$ 3.5225	$ 3.245	$ 2.915
Total cash distributions					$ 206,362	$ 202,942	$ 198,951	$ 194,961	$ 190,769	$ 186,783	$ 182,797	$ 178,808	$ 173,413	$ 168,296	$ 163,178	$ 158,061	$ 803,216	$ 739,157	$ 662,948
Subsequent Event [Member]
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation				170,604
Units granted			294,054
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			3 years
Partners' Capital Account, Distribution Per Unit of Limited Partner Interest		$ 0.920
Total cash distributions		$ 209,900
Management [Member]
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation																	216,679	350,552	354,529
Management [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation				168,913
Director [Member]
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation	[1]																23,961	6,117	4,461
Director [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Partners' Capital Account, Units, Unit-based Compensation				1,691

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