Subsequent Events (Details) (USD $)	0 Months Ended						6 Months Ended		12 Months Ended	0 Months Ended		12 Months Ended	0 Months Ended	1 Months Ended	0 Months Ended	6 Months Ended
	May 15, 2012	Feb. 14, 2012	Nov. 14, 2011	Aug. 12, 2011	May 13, 2011	Feb. 14, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Aug. 14, 2012 Scenario, Forecast [Member]		Dec. 31, 2012 Scenario, Forecast [Member]	Nov. 04, 2011 MF Global Holdings Ltd. Bankruptcy [Member]	Jul. 31, 2012 MF Global Holdings Ltd. Bankruptcy [Member]	Aug. 02, 2012 Forward-Starting Interest Rate Swaps [Member]	Jun. 30, 2012 Forward-Starting Interest Rate Swaps [Member]	Jul. 31, 2012 Forward-Starting Interest Rate Swaps [Member]
Subsequent Event [Line Items]
Interest rate swap agreement hedge															$ 250,000,000	$ 100,000,000	$ 150,000,000
Debt instrument, interest rate															6.45%
Debt instrument maturity date															Jun. 01, 2014
Fixed weighted-average semi-annual rate															2.60%	2.70%
Maximum hedge maturity, years															30 years	30 years
Amount received from existing trading positions													21,200,000	2,400,000
Remaining amount owed													8,200,000	5,800,000
Cash distribution per unit	$ 0.8400	$ 0.8150	$ 0.8000	$ 0.7850	$ 0.7700	$ 0.7575	$ 1.6550	$ 1.5275	$ 3.1125	$ 0.9425	[1]	$ 2.5975
Distributions declared							Jul. 26, 2012
Distribution date							Aug. 14, 2012
Distribution date of record							Aug. 07, 2012
Total cash distributions	$ 95,004,000	$ 92,177,000	$ 90,189,000	$ 88,498,000	$ 86,807,000	$ 85,398,000	$ 187,181,000	$ 172,205,000	$ 350,892,000	$ 106,597,000	[1]	$ 293,778,000

[1] Our general partner's board of directors declared this cash distribution on July 26, 2012 to be paid on August 14, 2012 to unitholders of record at the close of business on August 7, 2012.