Units Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Units Outstanding
Outstanding as of January 1, (in units)	96,652,190	100,044,485
Options exercised (in units)	1,179,860	358,262
Units issued (in units)	5,546,695	4,455,944
Units retired (in units)	(6,916,756)	(8,206,501)
Outstanding as of December 31, (in units)	96,461,989	96,652,190