Discontinued Operations (Details Textuals)	12 Months Ended
	Dec. 31, 2011 Properties	Dec. 31, 2010 Properties	Dec. 31, 2009 Properties
Discontinued Operations (Textuals) [Abstract]
Number of properties sold (in properties)	19	19	19
Discontinued Operations [Member]
Discontinued Operations (Textuals) [Abstract]
Net rentable square feet of properties sold (in square feet)	1,600,000