Commitments and Contingencies - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commitments And Contingencies [Line Items]
Operating ground lease expense	$ 162,000	$ 219,000	$ 486,000
Letter of credit obligations	4,900,000
Development in Process	248,602,000	88,848,000
Deferred Costs, Leasing, Gross	205,300,000	185,952,000
Maximum [Member]
Commitments And Contingencies [Line Items]
Potential payments for tenant allowances	27,300,000
Development Properties [Member]
Commitments And Contingencies [Line Items]
Deferred Costs, Leasing, Gross	7,600,000
Number of Development Buildings Under Commitment	10
Leasable space of buildings	3,400,000
Anticipated aggregate investment	315,700,000
Development Not Yet Commenced [Member]
Commitments And Contingencies [Line Items]
Anticipated aggregate investment	54,600,000
Land Improvements	4,500,000
Land Held for Development [Member]
Commitments And Contingencies [Line Items]
Purchase Commitment, Remaining Minimum Amount Committed	$ 34,000,000