Commitments and Contingencies (Details) (USD $)	1 Months Ended			9 Months Ended
	Aug. 31, 2010	Sep. 30, 2011 Year property segment	Sep. 30, 2011 Nonrecourse loan guarantees of unconsolidated partnership	Sep. 30, 2011 Arundel Preserve 5, LLC, Date Acquired 7/2/2007 sqfeet acre
Joint Ventures
Maximum approximate amount required to be paid under the guarantees			$ 65,000,000
Recovery percentage of amount paid under the guarantee when the entity continues to manage the properties (as a percent)			20.00%
Recovery percentage of amount paid under the guarantee when the entity no longer manages the properties (as a percent)			80.00%
Office space to be developed with a limited partnership (in square feet)				1,800,000
Land available to develop office space (in acres)				63
Ownership (as a percent)				50.00%
Tax incremental financing obligation
The amount of tax incremental financing bonds issued by Anne Arundel County, Maryland	30,000,000
Liability recognized with regard to tax incremental financing obligation at end of current period		4,300,000
Environmental indemnity agreement
Number of lease properties which were provided environmental indemnifications		3
Environmental indemnification to the tenant against losses covered under prior owner's indemnity agreement		5,000,000
Length of environmental indemnification to the tenant against consequential damages at a building after the tenant's acquisition of the property (in years)		15
Maximum environmental indemnification to the tenant against consequential damages after acquisition of property		12,500,000
Additional costs agreed to be paid by the entity related to construction and environmental regulatory activities (as a percent)		50.00%
Maximum annual additional costs agreed to be paid by the entity related to construction and environmental regulatory activities		300,000
Maximum additional costs agreed to be paid by the entity related to construction and environmental regulatory activities		$ 1,500,000