Consolidated Statements of Shareholders' Equity (USD $) In Thousands	Total USD ($)	Shares Outstanding [Member]	Shares of Beneficial Interest at Par Value [Member] USD ($)	Additional Paid in Capital [Member] USD ($)	Distributions in Excess of Net Income Attributable to the Controlling Interests [Member] USD ($)	Total Shareholders' Equity [Member] USD ($)	Noncontrolling Interests in Subsidiaries [Member] USD ($)
Balance at Dec. 31, 2011	$ 862,832		$ 662	$ 1,138,478	$ (280,096)	$ 859,044	$ 3,788
Balance, shares at Dec. 31, 2011		66,265
Net income attributable to the controlling interests	5,181				5,181	5,181
Distributions to noncontrolling interests	(7)						(7)
Contributions from noncontrolling interest	29						29
Dividends	(28,900)				(28,900)	(28,900)
Shares issued under Dividend Reinvestment Program	1,242		0	1,242		1,242
Shares issued under Dividend Reinvestment Program, shares		44
Share options exercised	23		0	23		23
Share options exercised, shares		0
Share grants, net of share grant amortization and forfeitures	1,319			1,319		1,319
Share grants, net of share grant amortization and forfeitures, shares		0
Balance at Mar. 31, 2012	$ 841,719		$ 662	$ 1,141,062	$ (303,815)	$ 837,909	$ 3,810
Balance, shares at Mar. 31, 2012		66,309