Investment in Unconsolidated Ventures (Narrative) (Details)	3 Months Ended
	Mar. 31, 2012 Properties Room Real_Estate_Ventures Buildings sqft A
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Number of unconsolidated Real Estate Ventures	18
Number of real estate ventures that owns 52 office buildings	15
Number of office buildings owned by 15 Real Estate Ventures	51
Net rentable square feet of 52 office buildings owned the 15 Real Estate Ventures	6,500,000
Number of real estate ventures that owns undeveloped land	2
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture	4
Number of real estate ventures developed hotel property	1
Number of rooms in a hotel property developed by one Real Estate Venture	137
Minimum ownership interest in unconsolidated Real Estate Ventures	20.00%
Maximum ownership interest in unconsolidated Real Estate Ventures	65.00%