Partnership Capital Subject to Mandatory Redemption - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 12, 2018	Dec. 31, 2014	Jan. 17, 2014
Partners Capital Account [Line Items]
Period of loans made by Partnership to general partners	1 year
Outstanding amount of partner loans financed through the Partnership	$ 297	$ 266	$ 218		$ 198
Interest income from outstanding amount of general partner loan	$ 13	$ 10	$ 8
Limited partnership's minimum annual return rate	7.50%	7.50%	7.50%
Limited partnership's minimum return, value	$ 67	$ 68	$ 69
2014 Limited Partnership Offering [Member]
Partners Capital Account [Line Items]
Limited partnership amount registered						$ 350
Limited partnership interests issued	298
Remaining limited partnership interests that may be issued	$ 52
2018 Limited Partnership Offering [Member] | Subsequent Event [Member]
Partners Capital Account [Line Items]
Limited partnership amount registered				$ 450