SCHEDULE IV - MORTGAGE LOANS ON REAL ESTATE (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 sqft	Dec. 31, 2011	Dec. 31, 2010	Jan. 17, 2013 sqft	Mar. 09, 2012 sqft	Feb. 10, 2012 sqft
Mortgage Loans on Real Estate [Line Items]
Carrying Amount of Mortgage	$ 0	$ 1,500,000
Area of real estate property (in square feet)	11,900,000			260,000	337,000	300,000
Movement in Mortgage Loans on Real Estate [Roll Forward]
Balance at beginning of year	1,500,000	10,336,000	8,126,000
Additions: [Abstract]
New mortgage loan	0	0	1,500,000
Amortization of discount	0	399,000	710,000
Deductions: [Abstract]
Mortgage Loans on Real Estate, Other Deductions	(1,500,000)
Non-cash impairment loss on mortgage loan receivable	0	(9,235,000)	0
Balance at end of year	0	1,500,000	10,336,000
One Park Ten [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Rate (in hundredths)	7.30%
Final Maturity Date	Jun. 01, 2012
Periodic Payment Term	Interest only (3)
Prior Liens	0
Face Amount of Mortgage	1,500,000
Principal Amount of Loan Subject to Delinquent Principal and Interest	0
Cost for federal tax purposes	0
2100 Ross, B Piece [Member]
Mortgage Loans on Real Estate [Line Items]
Interest Rate (in hundredths)	6.10%
Final Maturity Date	May 31, 2012
Periodic Payment Term	Interest only (2)
Prior Liens	0
Face Amount of Mortgage	10,000,000
Principal Amount of Loan Subject to Delinquent Principal and Interest	0
Original investment in mortgage loan	6,900,000
Cost for federal tax purposes	$ 0