Acquisitions and Dispositions (Summary of Significant Assumptions Utilized in Estimates) (Details) (USD $)	9 Months Ended
	Sep. 30, 2011 months
Minimum [Member]
Discount rate	6.40%
Exit capitalization rate	4.80%
Lease-up period (months)	12
Minimum [Member] | Industrial Buildings [Member]
Rental rate per square foot	2.75
Minimum [Member] | Office [Member]
Rental rate per square foot	8.61
Maximum [Member]
Discount rate	10.10%
Exit capitalization rate	9.00%
Lease-up period (months)	36
Maximum [Member] | Industrial Buildings [Member]
Rental rate per square foot	6.50
Maximum [Member] | Office [Member]
Rental rate per square foot	16.00