Commitments and Contingencies - Narrative (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Commitments And Contingencies [Line Items]
Operating ground lease expense	$ 41,000	$ 50,000	$ 121,000	$ 192,000
Letter of credit obligations	5,500,000		5,500,000
Development in Process	249,129,000		249,129,000		88,848,000
Potential payments for tenant allowances	12,600,000		12,600,000
Maximum [Member]
Commitments And Contingencies [Line Items]
Other Miscellaneous Guarantees Related to Unconsolidated Joint Ventures	118,000		118,000
Development Not Yet Commenced [Member]
Commitments And Contingencies [Line Items]
Land Improvements	6,100,000		6,100,000
Development in Progress [Member]
Commitments And Contingencies [Line Items]
Number of Real Estate Properties	14		14
Leasable space of buildings	3,600,000		3,600,000
Anticipated aggregate investment	349,100,000		349,100,000
Deferred Costs, Leasing, Gross	$ 6,900,000		$ 6,900,000