Investment in Unconsolidated Ventures (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 A Room sqft Real_Estate_Investments Real_Estate_Ventures Buildings		Nov. 30, 2010 Buildings
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	52
Net rentable square feet of 52 office buildings owned the 15 Real Estate Ventures	6,700,000
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%
Company's ownership percentage in the Real Estate Venture			25.00%
Percentage of interest acquired in partnerships by other partner			75.00%
Interest acquired in partnerships, Number			2
Number of office buildings owned			2
Real estate venture debt	$ 745,830,000
G&I Interchange Office LLC (DRA — N. PA) [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Company's ownership percentage in the Real Estate Venture	20.00%	[1],[2]
Sale of ownership in properties	80.00%
Real estate venture debt	181,967,000	[2]
Two Tower Bridge [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Company's ownership percentage in the Real Estate Venture	35.00%	[1],[3]
Real estate venture debt	13,482,000	[3]
Two Tower Bridge [Member] | Mortgage Maturing May 2013 [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Debt instrument stated interest rate	5.72%
Real estate venture debt	10,100,000
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Minimum	0.00%
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Maximum	1.00%
Two Tower Bridge [Member] | Mortgage Maturing May 2013 [Member] | Minimum [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Debt Instrument, Term	3 years
Two Tower Bridge [Member] | Mortgage Maturing May 2013 [Member] | Maximum [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Debt Instrument, Term	10 years
Two Tower Bridge [Member] | Municipal and state borrowings [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Real estate venture debt	3,400,000
Seven Tower Bridge [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Company's ownership percentage in the Real Estate Venture	20.00%	[1],[4]
Debt instrument stated interest rate	3.00%
Real estate venture debt	11,182,000	[4]
Debt Instrument, Number of Fixed Rate Mortgages	2
Seven Tower Bridge [Member] | Mortgage One and Two [Member] | Fixed Interest Rate Through February 2012 [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Debt instrument stated interest rate	4.00%
Seven Tower Bridge [Member] | Mortgage One and Two [Member] | Fixed Interest Rate Through February 2013 [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Debt instrument stated interest rate	5.00%
Seven Tower Bridge [Member] | Mortgage Maturing February 2014 [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Debt instrument stated interest rate	4.00%
Seven Tower Bridge [Member] | Secured Debt [Member] | Mortgage One Maturing February 2013 [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Real estate venture debt	7,000,000
Seven Tower Bridge [Member] | Secured Debt [Member] | Mortgage Two Maturing February 2013 [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Real estate venture debt	1,000,000
Seven Tower Bridge [Member] | Secured Debt [Member] | Mortgage Maturing February 2014 [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Real estate venture debt	2,000,000
Seven Tower Bridge [Member] | Line of Credit [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Real estate venture debt	1,200,000
Equity Method Investment Summarized Financial Information Debt, Maximum Borrowings	1,300,000
Coppell Associates [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Company's ownership percentage in the Real Estate Venture	50.00%	[1],[5],[6]
Real estate venture debt	18,060,000	[5],[6]
Debt Instrument, Number of Fixed Rate Mortgages	2
Debt Instrument, Blended Rate	5.89%
Coppell Associates [Member] | Senior Mortgage [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Debt instrument stated interest rate	5.75%
Real estate venture debt	16,400,000
Coppell Associates [Member] | Junior Mortgage [Member]
Investment in Unconsolidated Ventures (Textuals) [Abstract]
Debt instrument stated interest rate	6.89%
Real estate venture debt	$ 1,600,000

[1] Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.

[2] Proceeds received by the Company from the sale of an 80% ownership stake in the properties exceeded the historical cost of those properties. Accordingly, no investment in the real estate venture was reflected on the balance sheet at period end.

[3] Consists of a $10.1 million fixed rate mortgage with a 5.72% interest rate and May 2013 maturity date; also includes $3.4 million municipal and state borrowings with nominal interest rates (0 — 1%) and maturity dates from 3 to 10 years.

[4] Comprised of two fixed rate mortgages: $7.0 million and $1.0 million. Both mature in February 2013 and accrue interest at a fixed rate of 4% through February 2012, and 5% through February 2013. The total loan amount of $11.2 million as presented above also includes a 3% fixed rate loan (interest only through maturity) with a balance of $1.2 million as of December 31, 2011. This loan can be increased up to $1.3 million through its maturity date of September 2025. The remaining $2.0 million of the total amount features a 4% fixed rate with interest only through its maturity date of February 2014.

[5] Comprised of two fixed rate mortgages: a senior fixed rate note of $16.4 million at 5.75% that matures in March 2016 and a junior fixed rate note of $1.6 million at 6.89% that matures in December 2013; 5.89% is the blended rate.

[6] As a result of the adoption of the new accounting standard for the consolidation of variable interest entities, the Company deconsolidated these three real estate ventures as of January 1, 2010.