Significant Acquisitions and Dispositions (Summary of Significant Assumptions Utilized in Estimates) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 months	Dec. 31, 2010 months
Minimum [Member]
Discount rate	6.40%	8.90%
Exit capitalization rate	4.80%	7.60%
Lease-up period (months)	9	12
Minimum [Member] | Industrial Buildings [Member]
Rental rate per square foot	2.75	1.80
Minimum [Member] | Office [Member]
Rental rate per square foot	8.61	19.00
Minimum [Member] | Medical Office Properties [Member]
Rental rate per square foot	13.75	19.27
Maximum [Member]
Discount rate	11.10%	12.50%
Exit capitalization rate	10.00%	10.50%
Lease-up period (months)	36	36
Maximum [Member] | Industrial Buildings [Member]
Rental rate per square foot	6.70	8.00
Maximum [Member] | Office [Member]
Rental rate per square foot	16.00	19.00
Maximum [Member] | Medical Office Properties [Member]
Rental rate per square foot	27.62	19.27