Commitments and Contingencies - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Commitments And Contingencies [Line Items]
Operating ground lease expense	$ 40,000	$ 41,000
Letter of credit obligations	4,900,000
Potential payments for tenant allowances	38,500,000
Development in Process	209,690,000		248,602,000
Development Not Yet Commenced [Member]
Commitments And Contingencies [Line Items]
Land Improvements	4,400,000
Development in Progress [Member]
Commitments And Contingencies [Line Items]
Number of Real Estate Properties	12
Leasable space of buildings	3,700,000
Anticipated aggregate investment	302,300,000
Deferred Costs, Leasing, Gross	7,300,000
Land Held for Development [Member]
Commitments And Contingencies [Line Items]
Purchase Commitment, Remaining Minimum Amount Committed	$ 31,400,000