Significant Acquisitions and Dispositions (Summary of Significant Assumptions Utilized in Estimates) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 months	Dec. 31, 2011 months
Minimum [Member]
Fair Value Inputs, Discount Rate	7.13%	6.40%
Fair Value Inputs, Cap Rate	5.75%	4.80%
Lease-up period (months)	6	9
Minimum [Member] | Industrial [Member]
Rental rate per square foot	2.75	2.75
Minimum [Member] | Office [Member]
Rental rate per square foot		8.61
Minimum [Member] | Medical Office [Member]
Rental rate per square foot	13.20	13.75
Maximum [Member]
Fair Value Inputs, Discount Rate	10.78%	11.10%
Fair Value Inputs, Cap Rate	8.88%	10.00%
Lease-up period (months)	36	36
Maximum [Member] | Industrial [Member]
Rental rate per square foot	7.62	6.70
Maximum [Member] | Office [Member]
Rental rate per square foot		16.00
Maximum [Member] | Medical Office [Member]
Rental rate per square foot	26.14	27.62