Commitments and Contingencies - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 properties	Dec. 31, 2010	Dec. 31, 2009
Commitments And Contingencies [Line Items]
Operating ground lease expense	$ 322,000	$ 434,000	$ 460,000
Additional capital contributions to unconsolidated joint ventures	1,400,000
Letter of credit obligations	6,289,000
Development in Process	88,848,000	0
Number of Development Buildings Under Commitment	10
Maximum [Member]
Commitments And Contingencies [Line Items]
Other miscellaneous guarantees related to unconsolidated joint ventures	452,000
Potential payments for tenant allowances	38,100,000
Development Properties [Member]
Commitments And Contingencies [Line Items]
Leasable space of buildings	3,100,000
Anticipated aggregate investment	$ 288,700,000