Acquisitions (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Lease commisions [Member]	Mar. 31, 2013 Lease in place value [Member]	Mar. 31, 2013 Above market leases [Member]	Mar. 31, 2013 Below market leases [Member]	Mar. 31, 2013 Buildings [Member]	Mar. 31, 2013 Tenant Improvements [Member]	Mar. 31, 2013 US Airways Building [Member]	Mar. 15, 2013 US Airways Building [Member] sqft	May 03, 2013 Lincoln Place [Member]	Apr. 26, 2013 Lincoln Place [Member] sqft	Jan. 17, 2013 Tower Place 200 [Member] sqft	Mar. 07, 2013 Deerwood Portfolio [Member] sqft	Mar. 25, 2013 Tampa Fund II Assets [Member] sqft
Business Acquisition [Line Items]
Area of real estate property (in square feet)										225,000		140,000	258,000	1,000,000	788,000
Area of office space under contract												111,000
Area of retail space under contract												29,000
Number of stories in parking garage												5
Cost of entity acquired										$ 41,800,000			$ 56,300,000	$ 130,000,000	$ 97,500,000
Number of office properties purchased in one transaction														2	3
Preliminary allocation of purchase price [Abstract]
Land	31,337,000
Buildings & Garages	121,369,000
Tenant improvements	13,434,000
Lease commissions	6,056,000
Lease in place value	15,862,000
Above market leases	2,936,000
Below market leases	(5,735,000)
Ownership percentage of property (in hundredths)										75.00%
Leased percentage of a property acquisition										100.00%		100.00%
Number of stories in a property acquired										9
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners										25.00%					70.00%
Mortgage assumed in a business acquisition												49,600,000			40,700,000
Mortgage to refinance										18,000,000
Interest rate on mortgage (in hundredths)												5.90%		3.90%
Implied purchase price												66,000,000
closing stock price											$ 18.20
Operating partnership units issued												900,000
Maturity Date of Debt												June 11, 2016		April 1, 2023
First termination option										Dec. 31, 2016
Second termination option										Dec. 31, 2021
Months notice of termination									0 years 12 months 0 days
Mortgage secured for properties in a business acquisition														84,500,000
Expiration date of lease										April 2024
Mortgage loan on property in business combination															58,100,000
Property, Plant and Equipment [Line Items]
Weighted average life (in years)							40 years 0 months 0 days	5 years 0 months 0 days
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average life (in years)			4 years 0 months 0 days	4 years 0 months 0 days	4 years 0 months 0 days	7 years 0 months 0 days
Business Acquisition, Pro Forma Revenue	75,820,000	56,478,000
Business Acquisition, Pro Forma Net Income (Loss)	$ (4,307,000)	$ 2,430,000
Business Acquisition, Pro Forma Earnings Per Share, Basic	$ (0.08)	$ 0.11
Business Acquisition, Pro Forma Earnings Per Share, Diluted	$ (0.08)	$ 0.11