Partners' Capital And Distributions (Narrative) (Details) (USD $)	3 Months Ended							12 Months Ended	3 Months Ended				3 Months Ended				12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Waiver Units Class 1 [Member]	Mar. 31, 2013 Waiver Units Class 3 [Member]	Mar. 31, 2013 Waiver Units Class 4 [Member]	Dec. 31, 2012 Class A [Member]	Dec. 31, 2012 Class B [Member]	Mar. 31, 2013 Waiver Units [Member]	May 15, 2013 Waiver Units [Member]	Aug. 14, 2012 Waiver Units [Member]	Feb. 14, 2012 Waiver Units [Member]	Dec. 31, 2012 Class One [Member]	Mar. 31, 2013 Class Two [Member]	Dec. 31, 2012 Class Two [Member]	Mar. 31, 2013 Class Three [Member]	Dec. 31, 2012 Class Three [Member]	Mar. 31, 2013 Class Four [Member]	Dec. 31, 2012 Class Four [Member]	Mar. 31, 2013 Waiver Units Class2 [Member]
Partners Capital And Distributions [Line Items]
Common units, issued		81,202,752	81,202,752
Issuance of common units		0				5,750,000
Common units, outstanding		81,202,752	81,202,752			79,465,000	71,965,000				81,162,755	39,997
Waiver units authorized								1,738,000	1,750,000	1,750,000			3,476,466											1,750,000
Conversion Feature, Minimum Distribution Coverage Ratio		1.1																1.1		1.1		1.1
Waiver units issued									1,738,000	1,738,000														1,738,000
Conversion feature threshold, quarterly distribution, minimum, per unit																	0.43		0.46		0.49		0.52
Waiver Units Class 1 Convertible Date						February 14, 2012
Distribution paid per common unit		$ 0.4975	$ 0.4850	$ 0.4725	$ 0.4600	$ 0.4500								$ 0.50	$ 0.46	$ 0.44
Waiver Units Class 3 Convertible Date	May 15, 2013
Waiver units outstanding									5,214,099	5,214,099
Waiver Units Class 2 Convertible Date				August 14, 2012
Distribution Made to Member or Limited Partner, Distribution Date		May 15, 2013	Feb. 14, 2013	Nov. 14, 2012	Aug. 14, 2012	May 15, 2012