Commitments and Contingencies - Narrative (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Commitments And Contingencies [Line Items]
Operating ground lease expense	$ 41,000	$ 65,000	$ 81,000	$ 142,000
Letter of credit obligations	6,300,000		6,300,000
Potential payments for tenant allowances	24,900,000		24,900,000
Development in Process	198,472,000		198,472,000		88,848,000
Maximum [Member]
Commitments And Contingencies [Line Items]
Other miscellaneous guarantees related to unconsolidated joint ventures	229,000		229,000
Development Not Yet Commenced [Member]
Commitments And Contingencies [Line Items]
Anticipated aggregate investment	30,300,000		30,300,000
Land Improvements	2,400,000		2,400,000
Development in Progress [Member]
Commitments And Contingencies [Line Items]
Number of Real Estate Properties	13		13
Leasable space of buildings	3,300,000
Anticipated aggregate investment	310,100,000		310,100,000
Deferred Costs, Leasing, Gross	$ 6,300,000		$ 6,300,000