Mortgage Loans (Details) (USD $)	3 Months Ended	12 Months Ended				12 Months Ended		0 Months Ended
	Sep. 30, 2012	Dec. 31, 2013 sqft	Dec. 31, 2012	Dec. 31, 2011	Mar. 09, 2012 sqft	Dec. 31, 2013 2100 Ross, B Piece sqft	Dec. 31, 2013 One Park Ten	Jun. 03, 2013 US Airways Building [Member] sqft
Mortgage Loans on Real Estate [Line Items]
Face amount of mortgage note receivable						$ 10,000,000.0	$ 1,500,000.0	$ 3,500,000.0
Interest rate on mortgage (in percent						6.10%	7.30%	3.00%
Area of real estate property (in square feet)		17,600,000			337,000	844,000		225,000
Original investment in mortgage loan						6,900,000
Non-cash impairment loss on mortgage loan receivable		0	0	9,235,000
Carrying amount of mortgage loans receivable		3,502,000	0	0
Recovery of loss on mortgage loan receivable	$ 500,000	$ 0	$ 500,000	$ 0