Notes Payable (Other Debt Information Textuals) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Long Term Debt Nonrecourse	Dec. 31, 2012 Mahan Village	Sep. 30, 2011 Mahan Village sqft	Dec. 31, 2012 191 Peachtree Tower mortgage note	Mar. 31, 2012 191 Peachtree Tower mortgage note sqft
Debt Instrument [Line Items]
Debt instrument original principal amount							$ 100
Square footage of real estate property (square feet)					147,000		1,200,000
Interest rate on mortgage loan (percent)						3.35%
Construction loan maximum borrowing capacity				15.0
Adjusted interest rate for construction loan	LIBOR
Basis spread on variable rate (percent)	1.50%			1.65%
Effective debt interest rate (percentage)				1.86%
Maximum percentage guaranteed under construction loan (percent)				25.00%
Carrying value of assets pledged as collateral for debt			387.7
Mortgage notes payable			425.4
Weighted average maturity of consolidated debt			6 years 8 months 12 days
Estimated fair value of notes payable	$ 456.0	$ 568.5