Units Outstanding (Details)	3 Months Ended
	Mar. 31, 2018 shares
Units Outstanding
Outstanding as of December 31, 2017 (in units)	96,461,989
Options exercised (in units)	235,105
Units issued (in units)	949,185
Units retired (in units)	(2,536)
Outstanding as of March 31, 2018 (in units)	97,643,743