Partners' Capital And Distributions (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								6 Months Ended		6 Months Ended		6 Months Ended		0 Months Ended			6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012	Dec. 31, 2010	Jun. 30, 2012 Waiver Units Class 1 [Member]	Mar. 31, 2012 Waiver Units Class 1 [Member]	Jun. 30, 2012 Waiver Units Class 3 [Member]	Jun. 30, 2012 Waiver Units Class 4 [Member]	Mar. 28, 2012 Class A [Member]	Jun. 30, 2012 Class A [Member]	Jun. 30, 2012 Class B [Member]	Jun. 30, 2012 Waiver Units [Member]	Jun. 30, 2012 Class One [Member]	Jun. 30, 2012 Class One [Member]	Jun. 30, 2012 Class Two [Member]	Jun. 30, 2012 Class Two [Member]	Jun. 30, 2012 Class Three [Member]	Jun. 30, 2012 Class Three [Member]	Jun. 30, 2012 Class Four [Member]	Jun. 30, 2012 Class Four [Member]	Jun. 30, 2012 Waiver Units Class2 [Member]
Partners Capital And Distributions [Line Items]
Issuance of common units									5,750,000						5,750,000
Price per share of common units issued															$ 30.80
Proceeds received from offering, net of costs															$ 169.4
Common units, outstanding	79,464,519				71,965,062		64,615,000		79,464,519	64,615,000						79,424,522	39,997
Waiver units authorized											1,750,000		1,750,000	1,750,000													1,750,000
Waiver units issued											1,738,000		1,738,000	1,738,000													1,738,000
Conversion feature, minimum distribution coverage ratio																			1.1		1.1		1.1		1.1
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	[1]
Distribution paid per common unit	$ 0.4600		$ 0.4500		$ 0.4400	$ 0.4275	$ 0.4150	$ 0.4075	$ 0.4600			$ 0.44															$ 0.46
Waiver units outstanding													5,214,099	5,214,099				5,214,099
Waiver Units Class 2 Convertible Date	August 14, 2012	[1]
Distribution Made to Member or Limited Partner, Distribution Date	August 14, 2012	[1]	May 15, 2012		February 14, 2012	November 14, 2011	August 12, 2011	May 13, 2011

[1] (1) This distribution will be paid to unitholders of record as of August 1, 2012.