Equity (Details) (USD $) In Thousands, except Per Share data	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Equity Offering
Issuance of units in public offering, net of offering costs (in units)			3,450
Issuance of units in public offering, net of offering costs			$ 138,163	$ 142,255
Distributions of Available Cash
Distribution per common unit (in dollars per unit)	$ 0.70	$ 0.67			$ 0.65
Record Date	August1,2011	May2,2011			February7,2011
Payment Date	August12,2011	May13,2011			February14,2011