Distributions (Distributions) (Details) (USD $) In Thousands, except Per Share data	0 Months Ended									1 Months Ended	9 Months Ended		12 Months Ended
	Nov. 14, 2011		Aug. 12, 2011	May 13, 2011	Feb. 14, 2011	Nov. 12, 2010	Aug. 13, 2010	May 14, 2010	Feb. 12, 2010	Oct. 31, 2011	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2011	Dec. 31, 2010
Distributions [Abstract]
Per Unit Cash Distribution Amount	$ 0.8000	[1]	$ 0.7850	$ 0.7700	$ 0.7575	$ 0.7450	$ 0.7325	$ 0.7200	$ 0.7100	$ 0.800	$ 2.3125	$ 2.1625	$ 3.1125	$ 2.9075
Total Cash Distribution to Limited Partners	$ 90,189	[1]	$ 88,498	$ 86,807	$ 85,398	$ 83,798	$ 82,393	$ 76,847	$ 75,779	$ 90,200	$ 260,703	$ 235,019	$ 350,892	$ 318,817
Cash distribution paid date	11/14/2011(a)	[1]	8/12/2011	5/13/2011	2/14/2011	11/12/2010	8/13/2010	5/14/2010	2/12/2010	November 14, 2011	Through 9/30/2011	Through 9/30/2010
Cash distribution declared date										October 2011	October 18, 2011
Cash distribution record date										November 1, 2011	November 1, 2011

[1] Our general partner's board of directors declared this cash distribution on October 18, 2011 to be paid on November 14, 2011 to unitholders of record at the close of business on November 1, 2011.