Mortgage Loans (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Real Estate Properties [Line Items]
Recovery of losses on mortgage loan receivable	$ 500,000	$ 0	$ 500,000	$ 0
One Park Ten [Member]
Real Estate Properties [Line Items]
Face amount of mortgage note receivable	1,500,000		1,500,000
Interest rate on mortgage (in hundredths)			7.30%
2100 Ross, B Piece [Member]
Real Estate Properties [Line Items]
Face amount of mortgage note receivable	10,000,000		10,000,000
Interest rate on mortgage (in hundredths)			6.10%
Area of real estate property (in square feet)	844,000		844,000
Original investment in mortgage loan	6,900,000		6,900,000
Non-cash impairment loss on mortgage loan				9,200,000
Investment in mortgage loan	0		0
Recovery of losses on mortgage loan receivable			$ 500,000