Units Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Units Outstanding
Outstanding as of January 1, (in units)	96,658,278	96,461,989
Options exercised (in units)	511,894	889,119
Units issued (in units)	4,833,715	6,153,320
Units retired (in units)	(3,811,789)	(6,846,150)
Outstanding as of December 31, (in units)	98,192,098	96,658,278