Investment in Unconsolidated Ventures (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 sqft Buildings Room A Real_Estate_Ventures		Dec. 31, 2011
Schedule of Equity Method Investments [Line Items]
Company's share of equity (Company's basis)	$ 193,555		$ 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%
Six Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Company's share of equity (Company's basis)	$ 13,330
Six Tower Bridge [Member] | Priority Loan [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	9.00%	[1]
Debt Instrument, Priority	4.9	[1]

[1] (d)On November 1, 2012, Six Tower Bridge Associates paid off its $12.2 million loan using funds contributed by the Company in the form of a $8.4 million partner loan which pays current interest expense at 9.0%, and a $4.9 million priority loan which accrues interest at 9.0%, compounding annually (the additional funds contributed from the partner and priority loans were used for operations of the venture).