Commitments and Contingencies (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010	Sep. 30, 2011		Sep. 30, 2010
Loss Contingencies [Line Items]
Commitment to Provide Funding to Tenants for Capital Improvements	$ 145,300,000			$ 145,300,000
Commitment To Provide Funding To Tenants For Capital Improvements Period	5 years
Number of Lawsuits as Defendant	2			2
Letter of Credit Renewal Period	1 year
Secured Debt [Member] | W Wacker Building 35 [Member]
Loss Contingencies [Line Items]
Commitment to Provide Funding to Tenants for Capital Improvements	34,600,000			34,600,000
Unsecured Debt [Member] | Unsecured $500 Million Facility [Member]
Loss Contingencies [Line Items]
Debt Instrument, Face Amount	500,000,000			500,000,000
Letter Of Credit August 2012 [Member]
Loss Contingencies [Line Items]
Letters of Credit Outstanding, Amount	382,556	[1]		382,556	[1]
Letter of Credit 2, February 2012 [Member]
Loss Contingencies [Line Items]
Letters of Credit Outstanding, Amount	14,782,820	[1]		14,782,820	[1]
Letter of Credit, December 2011 [Member]
Loss Contingencies [Line Items]
Letters of Credit Outstanding, Amount	2,006,589	[1]		2,006,589	[1]
Letter Of Credit, June 2012 [Member]
Loss Contingencies [Line Items]
Letters of Credit Outstanding, Amount	9,033,164	[1]		9,033,164	[1]
Collectibility of Tenant Reimbursements [Member]
Loss Contingencies [Line Items]
Loss Contingency Accrual, Carrying Value, Provision	$ 100,000		$ 45,000	$ 100,000		$ 100,000

[1] These letter of credit agreements automatically renew for consecutive, one-year periods each anniversary, subject to the satisfaction of the credit obligation and certain other limitations.