Equity (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	0 Months Ended									3 Months Ended												12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended		12 Months Ended			1 Months Ended				0 Months Ended		12 Months Ended
	Nov. 19, 2012	Aug. 17, 2012	May 14, 2012	Mar. 16, 2012	Jan. 13, 2012	Dec. 19, 2011	Oct. 13, 2011	Jul. 13, 2011	Jan. 14, 2011	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Nov. 30, 2012 November 2012 ATM	Dec. 31, 2013 November 2012 ATM	Dec. 31, 2012 November 2012 ATM	Aug. 31, 2013 August 2013 ATM	Sep. 30, 2013 September 2013 ATM	Dec. 31, 2013 Common Units	Dec. 31, 2012 Common Units	Dec. 31, 2011 Common Units	Aug. 31, 2013 Common Units August 2013 ATM	Sep. 30, 2013 Common Units September 2013 ATM	Dec. 31, 2013 Class B Units	Dec. 31, 2012 Class B Units	Jul. 02, 2014 Class B Units M & R anniversary installment	Jul. 02, 2013 Class B Units M & R	Dec. 31, 2013 Maximum
Equity
Period after the end of each quarter within which available cash is distributed to unitholders																																							45 days
Period for which cash reserves are created for distribution to unitholders and the general partner																																							1 year
Equity Offerings
Issuance of units in public equity offerings, net of offering costs (in units)	9,800	6,900	8,000	6,800	700	10,000	5,800	4,000	3,500																9,400	9,300	100	5,900	10,900	26,115	32,308	23,225
Proceeds from public equity offerings, net	$ 437,000,000	$ 338,000,000	$ 427,000,000	$ 388,000,000	$ 38,000,000	$ 521,000,000	$ 251,000,000	$ 185,000,000	$ 138,000,000													$ 1,698,066,000	$ 1,634,081,000	$ 1,095,488,000	$ 590,000,000	$ 584,000,000	$ 6,000,000	$ 396,000,000	$ 718,000,000
Maximum amount of limited partner interests that can be sold through the Manager																									$ 600,000,000								$ 400,000,000	$ 1,000,000,000
Common units																														157,766	127,494				15,964	19,954		4,000
Conversion ratio																																						1
Number of installments																																					4
Number of anniversaries																																					3
Distributions of Available Cash
Distribution per common unit (in dollars per unit)										$ 0.86	$ 0.85	$ 0.84	$ 0.83	$ 0.82	$ 0.81	$ 0.80	$ 0.79	$ 0.76	$ 0.73	$ 0.70	$ 0.67