Real Estate Investments (Dispositions) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			0 Months Ended				0 Months Ended
	Mar. 31, 2011	Dec. 31, 2011 A	Nov. 22, 2011 Four Four Two Creamery Way [Member] sqft	Oct. 28, 2011 Five Greentree Center and Lake Center II [Member] Properties sqft	Oct. 28, 2011 Five Greentree Centre [Member]	Oct. 28, 2011 Lake Center II [Member]	Jun. 27, 2011 Three Greentree Center [Member] sqft	Dec. 23, 2010 One and Two Greentree Centre, 8000 Lincoln Drive, and Lake Center IV [Member] Properties sqft	Nov. 22, 2010 Spyglass Point [Member] sqft	Sep. 20, 2010 630 Clark Avenue [Member] sqft	Aug. 18, 2010 479 Thomas Jones Way [Member] sqft	Jan. 14, 2010 Westmoreland Plaza [Member] sqft	Oct. 14, 2011 Dallas, Texas [Member] A
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of office properties sold (in properties)				2				4
Net rentable square feet of properties sold			104,500	206,243			69,300	243,195	58,576	50,000	49,264	121,815
Sales price of properties sold			$ 7.4	$ 22.8			$ 5.9	$ 20.9	$ 13.5	$ 3.6	$ 3.8	$ 10.8	$ 0.6
Lease percentage of sold property			100.00%		92.20%	47.80%	13.90%	76.10%			63.00%
Area of undeveloped parcels of land		4											6
Gain on sale of ownership interest	$ 2.8
Number of properties with ownership interest	3
Remaining ownership interest in real estate properties	11.00%