Acquisitions and Dispositions (Summary of Significant Assumptions Utilized in Estimates) (Details) (USD $)	6 Months Ended
	Jun. 30, 2011
Industrial Buildings [Member] | Minimum [Member]
Rental rate per square foot	3.30
Office [Member] | Minimum [Member]
Rental rate per square foot	8.61
Minimum [Member]
Discount rate	6.40%
Exit capitalization rate	4.80%
Lease- up period (months)	12
Industrial Buildings [Member] | Maximum [Member]
Rental rate per square foot	6.50
Office [Member] | Maximum [Member]
Rental rate per square foot	16.00
Maximum [Member]
Discount rate	10.00%
Exit capitalization rate	8.70%
Lease- up period (months)	36