Capital and Financing Transactions (Details) (USD $)			3 Months Ended			3 Months Ended					3 Months Ended						3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Phoenix Tower [Member]	Feb. 21, 2013 Phoenix Tower [Member] sqft	Mar. 25, 2013 Tampa Fund II Assets [Member]	Mar. 31, 2013 Deerwood Portfolio [Member]	Mar. 07, 2013 Deerwood Portfolio [Member]	Mar. 31, 2013 Hayden Ferry Lakeside II [Member] sqft	Mar. 31, 2013 Mortgages [Member]	Mar. 31, 2013 Term Loan Swap [Member]	Mar. 31, 2013 Unsecured Working Capital Revolving Credit Facility [Member]	Mar. 31, 2013 Unsecured Working Capital Revolving Credit Facility [Member] Minimum [Member]	Mar. 31, 2013 Unsecured Working Capital Revolving Credit Facility [Member] Maximum [Member]	Mar. 31, 2013 Unsecured Term Loan [Member]	Mar. 31, 2013 Unsecured Term Loan [Member] Minimum [Member]	Mar. 31, 2013 Unsecured Term Loan [Member] Maximum [Member]	Mar. 31, 2013 Wells Fargo [Member] Unsecured Revolving Credit Facility [Member]		Mar. 31, 2013 Key Bank [Member] Unsecured Term Loan [Member]		Mar. 31, 2013 PNC Bank [Member] Unsecured Working Capital Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Interest Rate	2.20%									0.70%							0.00%	[1]	2.20%	[2]	0.00%	[1]
Maturity																	Mar. 29, 2016		Sep. 27, 2017		Mar. 29, 2016
Outstanding Balance	$ 125,000,000																$ 0		$ 125,000,000		$ 0
Variable rate basis											LIBOR
Basis spread on variable rate (in hundredths)												1.60%	2.35%		1.50%	2.30%
LIBOR rate period fixed											1.60%			1.50%
Commitment fee (in hundredths)												0.25%	0.35%
Commitment Fee for the current period (in hundredths)											0.35%
Notional amount	125,000,000
Term Loan Swap Term										5 years 0 months 0 days
Debt Instrument [Line Items]
Mortgage notes payable	768,005,000	605,890,000
Mortgage notes payable footnote disclosure				80,000,000	40,700,000		84,500,000		768,000,000
Weighted average interest rate (in hundredths)									5.00%
Interest rate on mortgage (in hundredths)				3.90%			3.90%
Date at which mortgage loan is pre-payable				Mar. 01, 2015			May 01, 2015
Maturity Date of Debt				March 1, 2023			April 1, 2023
Interest only period of mortgage (in months)			2 years 0 months 0 days			3 years 0 months 0 days
Proportionate share of mortgage loan					$ 58,100,000
Area of real estate property (in square feet)				629,000				299,000

[1] The interest rate on the credit facilities is based on LIBOR plus an applicable margin of 160 to 235 basis points, depending upon overall Company leverage as defined in the loan agreements for the Company's credit facilities, with the current rate set at 160 basis points. Additionally, the Company pays fees on the unused portion of the credit facilities ranging between 25 and 35 basis points based upon usage of the aggregate commitment, with the current rate set at 35 basis points.

[2] The interest rate on the term loan is based on LIBOR plus an applicable margin of 150 to 230 basis points depending on overall Company leverage (with the current rate set at 150 basis points). On September 28, 2012, the Company executed two floating-to-fixed interest rate swaps totaling $125 million, locking LIBOR at 0.7% for five years which was effective October 1, 2012.