Members' Equity and Net Income per Common and Class B Unit - Common and Class B Units Rollforward (Details) - Common Units - shares shares in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Increase (Decrease) in Members' Equity [Roll Forward]
Beginning of period (shares)	83,452	78,337
Issuance of Common units for cash (shares)	2,244	4,864
Repurchase of units under the Common unit buyback program (shares)	(157)	(135)
Reissuance of Common units for restricted unit grants (shares)	288	0
Unit-based compensation (shares)	578	386
End of period (shares)	86,405	83,452