Equity (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended			1 Months Ended		5 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Sep. 30, 2013	Nov. 30, 2014	Mar. 31, 2015
Equity Offerings
Proceeds from public equity offerings, net	$ 0	$ 271,880,000
Common Units
Equity Offerings
Aggregate common units sold (in units)		4,249
Distributions of Available Cash
Distribution per common unit (in dollars per unit)	$ 0.91		$ 0.90
Common Units | ATM
Equity Offerings
Aggregate common units sold (in units)		4,200
Proceeds from public equity offerings, net		272,000,000
Common Units | September 2013 ATM
Equity Offerings
Aggregate common units sold (in units)		4,200
Proceeds from public equity offerings, net		272,000,000
Offering under program				1,000,000,000
Manager fees		4,000,000
Common Units | November 2014 ATM
Equity Offerings
Aggregate common units sold (in units)						2,500
Proceeds from public equity offerings, net						175,000,000
Offering under program					$ 1,500,000,000
Class B Units | M & R
Equity Offerings
Conversion ratio	1
Number of installments	3
Number of anniversaries	2
Maximum | Common Units
Distributions of Available Cash
Common Unit Price (in dollars per share)	$ 69.16		$ 77.31			$ 69.16
Minimum | Common Units
Distributions of Available Cash
Common Unit Price (in dollars per share)	$ 54.04		$ 58.67			$ 54.04