Units Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Units Outstanding
Outstanding as of January 1, (in units)	100,044,485	100,756,999
Options exercised (in units)	358,262	541,073
Units issued (in units)	4,455,944	4,600,583
Units retired (in units)	(8,206,501)	(5,854,170)
Outstanding as of December 31, (in units)	96,652,190	100,044,485