Distributions (Distributions) (Details) (USD $) In Thousands, except Per Share data	0 Months Ended								1 Months Ended	3 Months Ended	6 Months Ended		12 Months Ended
	Aug. 12, 2011		May 13, 2011	Feb. 14, 2011	Nov. 12, 2010	Aug. 13, 2010	May 14, 2010	Feb. 12, 2010	Jul. 31, 2011	Jun. 30, 2011	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2011	Dec. 31, 2010
Distributions
Per Unit Cash Distribution Amount	$ 0.7850	[1]	$ 0.7700	$ 0.7575	$ 0.7450	$ 0.7325	$ 0.7200	$ 0.7100	$ 0.785	$ 1.5275	$ 1.5275	$ 1.4300	$ 2.3125	$ 2.9075
Total Cash Distribution to Limited Partners	$ 88,498	[1]	$ 86,807	$ 85,398	$ 83,798	$ 82,393	$ 76,847	$ 75,779	$ 88,500		$ 172,205	$ 152,626	$ 260,703	$ 318,817
Cash distribution paid date	8/12/2011(a)	[1]	5/13/2011	2/14/2011	11/12/2010	8/13/2010	5/14/2010	2/12/2010	August 12, 2011	August 12, 2011	Through 6/30/2011	Through 6/30/2010
Cash distribution declared date									July 2011		July 21, 2011
Cash distribution record date									August 4, 2011		August 4, 2011

[1] (a) Our general partner's board of directors declared this cash distribution on July 21, 2011 to be paid on August 12, 2011 to unitholders of record at the close of business on August 4, 2011.