Shareholders' Equity (Details) (USD $) In Thousands, except Share data	1 Months Ended				9 Months Ended		12 Months Ended
	Oct. 31, 2011	Aug. 31, 2011	Jul. 31, 2011	Nov. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 16, 2011	Oct. 04, 2011 Distribution declared in October 2011
Distributions
Distribution to common shareholders (in dollars per share)		$ 0.37		$ 0.37
Distribution to common shareholders		$ 56,748		$ 47,167	$ 161,724	$ 137,589
Distribution declared to common shareholders (in dollars per share)									$ 0.38
Distribution declared to common shareholders									61,805
Common shares of beneficial interest
Common shares of beneficial interest granted (in shares)								72,450
Common shares of beneficial interest, par value (in dollars per share)					$ 0.01		$ 0.01	$ 0.01
Common stock price per share which is the closing price at New York Stock Exchange (in dollars per share)								$ 23.90
Common shares issued in public offering (in shares)	9,200,000		11,500,000		11,500,000		14,400,000
Net proceeds from issuance of common shares issued in public offering	$ 184,739		$ 247,500		$ 247,499