Equity (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended							0 Months Ended			0 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Mar. 31, 2014 Common Units	Mar. 31, 2013 Common Units	Mar. 31, 2014 Common Units September 2013 ATM	Mar. 31, 2014 Common Units March 2014 ATM	Jul. 01, 2014 Class B Units M & R Subsequent events item	Mar. 31, 2014 Maximum	Dec. 31, 2013 Maximum	Sep. 05, 2013 Maximum Common Units September 2013 ATM	Mar. 11, 2014 Maximum Common Units March 2014 ATM	Mar. 31, 2014 Minimum	Dec. 31, 2013 Minimum
Equity
Offering under program											$ 1,000,000,000	$ 1,200,000,000
Equity Offerings
Aggregate common units sold (in units)				4,249	1,921	4,200	0
Proceeds from public equity offerings, net	271,880,000		103,937,000			272,000,000
Manager fees						$ 3,500,000
Conversion ratio								1
Number of installments								4
Number of anniversaries								3
Common Unit Price (in dollars per share)									$ 73.42	$ 75.79			$ 61.60	$ 62.56
Distributions of Available Cash
Distribution per common unit (in dollars per unit)	$ 0.87	$ 0.86