Mortgage Loans (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012 sqft	Dec. 31, 2011
One Park Ten [Member]
Real Estate Properties [Line Items]
Face amount of mortgage note receivable	$ 1.5
Interest rate on mortgage (in hundredths)	7.30%
Investment in mortgage loan	0
2100 Ross, B Piece [Member]
Real Estate Properties [Line Items]
Face amount of mortgage note receivable	10.0
Interest rate on mortgage (in hundredths)	6.10%
Area of real estate property (in square feet)	844,000
Original investment in mortgage loan	6.9
Non-cash impairment loss on mortgage loan		9.2
Investment in mortgage loan	$ 0