Partners' Capital (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended		4 Months Ended
	Feb. 28, 2018	Mar. 31, 2018	Mar. 31, 2017	Apr. 23, 2018
Business Acquisition [Line Items]
Proceeds from sales of common units		$ 0	$ 2,178
Cash distributions per common unit		$ 0.614	$ 0.60
Dividend Reinvestment Program [Member]
Business Acquisition [Line Items]
Shares Issued, Price Per Share	$ 38.83
Partners' Capital Account, Units, Sold in Private Placement	576,923
Proceeds from sales of common units	$ 22
Subsequent Event [Member]
Business Acquisition [Line Items]
Cash distributions per common unit				$ 0.614
Distribution Made to Limited Partner, Distribution Date				May 11, 2018
Class B Units Issued In Lieu Of Cash Distributions				318,553