Partnership Equity and Distributions - Cash Distribution (Detail) - USD ($) $ / shares in Units, $ in Millions																						3 Months Ended
	Sep. 15, 2020	Aug. 14, 2020	Jul. 15, 2020	Jun. 15, 2020	May 15, 2020	Apr. 15, 2020	Mar. 16, 2020	Feb. 14, 2020	Jan. 15, 2020	Dec. 16, 2019	Nov. 14, 2019	Oct. 15, 2019	Sep. 16, 2019	Aug. 14, 2019	Jul. 15, 2019	Jun. 17, 2019	May 15, 2019	Apr. 15, 2019	Mar. 15, 2019	Feb. 14, 2019	Jan. 15, 2019	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019
Distribution Made to Limited Partner [Line Items]
Total Cash Distribution																						$ 88	$ 104	$ 167	$ 160	$ 178	$ 159
Series A Preferred Limited Partners [Member]
Distribution Made to Limited Partner [Line Items]
Preferred Limited Partnership Distribution; Distribution Amount Paid				$ 36.875						$ 36.875						$ 36.875
Total Cash Distribution				$ 18						$ 19						$ 18						0	18	0	0	18	0
Series B Preferred Limited Partners [Member]
Distribution Made to Limited Partner [Line Items]
Preferred Limited Partnership Distribution; Distribution Amount Paid	$ 0.4922			$ 0.4922			$ 0.4922			$ 0.4922			$ 0.4922			$ 0.4922			$ 0.4922
Total Cash Distribution	$ 3			$ 3			$ 3			$ 4			$ 3			$ 3			$ 3			3	3	3	3	3	3
Series C Preferred Limited Partners [Member]
Distribution Made to Limited Partner [Line Items]
Preferred Limited Partnership Distribution; Distribution Amount Paid			$ 0.4969			$ 0.4969			$ 0.4969			$ 0.4969			$ 0.4969			$ 0.4969			$ 0.5576
Total Cash Distribution			$ 3			$ 2			$ 2			$ 2			$ 3			$ 2			$ 2	3	2	2	3	2	2
Limited Partners
Distribution Made to Limited Partner [Line Items]
Per Unit Distribution (in dollars per share)		$ 0.39			$ 0.39			$ 0.78			$ 0.78			$ 0.78			$ 0.78			$ 0.78
Total Cash Distribution		$ 82			$ 81			$ 162			$ 155			$ 154			$ 155			$ 154		$ 82	$ 81	$ 162	$ 112	$ 112	$ 111