Commitments and Contingencies - Narrative (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Commitments And Contingencies [Line Items]
Operating ground lease expense	$ 42,000	$ 41,000	$ 84,000	$ 81,000
Letter of credit obligations	4,900,000		4,900,000
Potential payments for tenant allowances	31,600,000		31,600,000
Development in Process	229,570,000		229,570,000		248,602,000
Development Not Yet Commenced [Member]
Commitments And Contingencies [Line Items]
Land Improvements	11,800,000		11,800,000
Development in Progress [Member]
Commitments And Contingencies [Line Items]
Number of Real Estate Properties	16		16
Leasable space of buildings	5,800,000		5,800,000
Anticipated aggregate investment	410,600,000		410,600,000
Deferred Costs, Leasing, Gross	9,300,000		9,300,000
Land Held for Development [Member]
Commitments And Contingencies [Line Items]
Purchase Commitment, Remaining Minimum Amount Committed	$ 42,400,000		$ 42,400,000