Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands								6 Months Ended		9 Months Ended	12 Months Ended
	Aug. 14, 2018	May 15, 2018	Feb. 14, 2018	Nov. 14, 2017	Aug. 14, 2017	May 15, 2017	Feb. 14, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Cash distribution per unit (in dollars per share)		$ 0.9375	$ 0.9200	$ 0.9050	$ 0.8900	$ 0.8725	$ 0.855	$ 1.8575	$ 1.7275		$ 3.5225
Total cash distributions		$ 213,933	$ 209,940	$ 206,362	$ 202,942	$ 198,951	$ 194,961	$ 423,873	$ 393,912		$ 803,216
Scenario, Forecast
Subsequent Event [Line Items]
Cash distribution per unit (in dollars per share)	$ 0.9575									$ 2.8150
Total cash distributions	$ 218,497									$ 642,370