Capital and Financing Transactions - Mortgage Notes Payable (Details) - USD ($) $ in Thousands						3 Months Ended		9 Months Ended
	Jul. 16, 2015	Apr. 08, 2015	Apr. 06, 2015	Apr. 03, 2015	Mar. 05, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Debt Instrument [Line Items]
Gain on extinguishment of debt						$ (702)	$ 0	$ (5,542)	$ 0
Mortgage debt								159,160	$ 42,203
Construction loan payable balance						18,900		18,900
Mortgages | Parent Company | 245 Riverside
Debt Instrument [Line Items]
Prepayment fee and swap early termination fee	$ 210
Mortgages | Parent Company | Two Ravinia Drive
Debt Instrument [Line Items]
Prepayment fee and swap early termination fee		$ 525
Westshore Corporate Center | Mortgages
Debt Instrument [Line Items]
Payoff balance					$ 14,000
Gain on extinguishment of debt					$ 79
Hillsboro I-V and V | Mortgages
Debt Instrument [Line Items]
Mortgage debt				$ 68,000
Prepayment fee and swap early termination fee				$ 3,000
3350 Peachtree | Mortgages
Debt Instrument [Line Items]
Mortgage debt			$ 31,900
Prepayment fee and swap early termination fee			$ 319
Two Ravinia Drive | Mortgages | Fund II
Debt Instrument [Line Items]
Mortgage debt		22,100
Prepayment fee and swap early termination fee		$ 1,800
245 Riverside | Mortgages | Fund II
Debt Instrument [Line Items]
Mortgage debt	9,100
Prepayment fee and swap early termination fee	$ 702
Hayden Ferry Lakeside III | Fund II
Debt Instrument [Line Items]
Proceeds from construction loans								18,400
Mortgages
Debt Instrument [Line Items]
Mortgage notes payable						1,200,000		1,200,000
Unamortized net premiums on debt						$ 19,000		$ 19,000
Weighted average interest rate (as a percent)						4.10%		4.10%