Commitments and Contingencies (Details Textual) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2012 litigation_settlement property	Dec. 31, 2012 sqft property	Dec. 31, 2011 property	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Anticipated amount of investment	$ 39,700,000	$ 39,700,000
Number of properties of a company developed	12	12	10
Properties leased	60.00%	60.00%	40.00%
Properties occupied	41.00%	41.00%	21.00%
Number of properties placed into service		2
Number of ground leases related to real estate investments under operating lease agreements	43	43
Operating lease expiration period maximum expiration date		2101
Approximately corporate office lease covers of rented space		30,934
Corporate lease expiration date		Oct. 31, 2020
Percentage on increases of annual base rent on the corporate office lease	3.25%	3.25%
Rental expense relating to the operating leases		4,300,000	4,300,000	4,000,000
Prepaid ground leases which represented rental expense		500,000	400,000	300,000
Number of claims settled	1
General and Administrative Expense [Member]
Property, Plant and Equipment [Line Items]
Litigation expense	950,000
Colorado [Member]
Property, Plant and Equipment [Line Items]
Number of buildings under construction commenced operations		1
Percentage of buildings leased	92.00%	92.00%
Texas [Member]
Property, Plant and Equipment [Line Items]
Number of buildings under construction commenced operations		1
Percentage of buildings leased	74.00%	74.00%
Construction mortgage loans [Member]
Property, Plant and Equipment [Line Items]
Number of Properties	2	2
Estimated remaining budget	$ 84,173,000	$ 84,173,000
Stabilization in Progress [Member]
Property, Plant and Equipment [Line Items]
Number of Properties	12	12