Units Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in Limited Partnership Units Outstanding
Outstanding as of January 1, (in units)	268,893,534	272,301,827
Options exercised (in units)	1,179,860	358,262
Units issued (in units)	5,546,695	4,455,944
Units retired (in units)	(6,960,756)	(8,222,499)
Outstanding as of December 31, (in units)	268,659,333	268,893,534
AB Units [Member]
Changes in Limited Partnership Units Outstanding
Units retired (in units)	(44,000)	(15,998)