Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Total Revenues and Other Income	$ 432	$ 411	$ 401	$ 423	$ 393	$ 384	$ 354	$ 355	$ 1,667	$ 1,486	$ 1,169
Income Before Income Taxes	255	238	234	199	223	217	186	174	926	800	528
Net Income Attributable to the Partnership	255	237	233	198	221	217	186	172	923	796	461
Limited Partners’ Interest in Net Income Attributable to the Partnership	$ 246	$ 228	$ 153	$ 119	$ 144	$ 144	$ 121	$ 110	$ 746	$ 519	$ 292
Net Income Attributable to the Partnership Per Common Unit—Basic (in usd per share)	$ 1.08	$ 1.18	$ 1.23	$ 0.96	$ 1.16	$ 1.17	$ 0.99	$ 0.91
Net Income Attributable to the Partnership Per Common Unit—diluted (in usd per share)	$ 1.06	$ 1.15	$ 1.15	$ 0.92	$ 1.09	$ 1.10	$ 0.94	$ 0.87