Investment in Unconsolidated Ventures (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 A Properties Room sqft Real_Estate_Ventures Buildings	Dec. 31, 2011
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Company's share of equity (Company's basis)	$ 179,037	$ 115,807
Number of unconsolidated Real Estate Ventures	19
Number of real estate ventures that owns 51 office buildings	15
Number of office buildings owned by 15 Real Estate Ventures	54
Net rentable square feet of 51 office buildings owned the 15 Real Estate Ventures	7,000,000
Number of real estate ventures that owns undeveloped land	3
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture	24
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%