Subsequent Events (Details) (USD $)	3 Months Ended		6 Months Ended		0 Months Ended		0 Months Ended	1 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jul. 03, 2014 Subsequent Event [Member] 3.50% (3.34% effective rate) Mortgage Loan due 2015 [Member]	Jul. 02, 2014 Subsequent Event [Member] 3.50% (3.34% effective rate) Mortgage Loan due 2015 [Member]	Jul. 02, 2014 Subsequent Event [Member] Board of Trade Investment Company [Member]	Jul. 29, 2014 Subsequent Event [Member] Raleigh, NC Office Properties Disposition (7/2014) [Member] property
Subsequent Event [Line Items]
Cash received related to redemption of investment in unconsolidated affiliate							$ 4,700,000
Early repayment of debt					36,900,000
Current interest rate (in hundredths)						3.34%
Debt instrument maturity date					Apr. 01, 2015
Loss on debt extinguishment	18,000	0	18,000	(164,000)	300,000
Number of office properties sold								5
Proceeds received from sale of real estate								58,700,000
Gain on disposition of property	$ 5,947,000	$ (37,000)	$ 5,947,000	$ (37,000)				$ 11,700,000