Commitments and Contingencies - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Commitments And Contingencies [Line Items]
Operating ground lease expense	$ 96,000	$ 77,000
Letter of credit obligations	6,289,000
Potential payments for tenant allowances	23,900,000
Development in Process	138,634,000		88,848,000
Maximum [Member]
Commitments And Contingencies [Line Items]
Other miscellaneous guarantees related to unconsolidated joint ventures	350,000
Development Not Yet Commenced [Member]
Commitments And Contingencies [Line Items]
Anticipated aggregate investment	6,700,000
Land Improvements	2,100,000
Development in Progress [Member]
Commitments And Contingencies [Line Items]
Number of Real Estate Properties	11
Leasable space of buildings	3,200,000
Anticipated aggregate investment	294,300,000
Deferred Costs, Leasing, Gross	$ 5,200,000