Investments in Unconsolidated Entities (Details) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended				0 Months Ended			1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Unconsolidated entities	Sep. 30, 2011 Unconsolidated entities	Sep. 30, 2012 Wanamaker Building	Dec. 31, 2011 Wanamaker Building	Oct. 16, 2012 Alamo Plaza	Sep. 30, 2012 Alamo Plaza	Dec. 31, 2011 Alamo Plaza	Feb. 29, 2012 St. Louis Place	Sep. 30, 2012 St. Louis Place	Dec. 31, 2011 St. Louis Place	Sep. 30, 2012 200 South Wacker	Dec. 31, 2011 200 South Wacker
Investments in unconsolidated entities
Ownership Interest (as a percent)								60.00%	60.00%		40.66%	40.66%		0.00%	35.71%	9.86%	9.82%
Amount of investment	$ 65,919,000		$ 65,919,000		$ 71,280,000			$ 48,985,000	$ 48,170,000		$ 13,831,000	$ 13,992,000			$ 6,304,000	$ 3,103,000	$ 2,814,000
Gain on troubled debt restructuring			3,600,000	4,600,000									200,000
Gain on transfer of assets													400,000
Amount of equity in earnings (losses) recognized by the entity	398,000	(2,779,000)	1,327,000	(2,186,000)
Distributions from investments						2,800,000	1,100,000
Additional investments in unconsolidated entities			730,000	4,057,000
Ownership interest in the total contract sales price										21,800,000
Proceeds from Sale										$ 8,800,000