Notes Payable (Other Information) (Details) - USD ($)	1 Months Ended				2 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2017	Jun. 30, 2017	May 31, 2017	Apr. 30, 2017	May 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Debt Disclosure [Abstract]
Total interest incurred						$ 10,288,000	$ 8,939,000	$ 32,360,000	$ 25,446,000
Less interest - discontinued operations						0	(1,956,000)	0	(5,896,000)
Interest capitalized						(2,701,000)	(1,229,000)	(6,509,000)	(2,988,000)
Total interest expense						7,587,000	5,754,000	25,851,000	16,562,000
Extinguishment of Debt [Line Items]
Gain (loss) on extinguishment of debt					$ 2,300,000	$ 429,000	$ 0	$ 2,258,000	$ 0
Mortgage Debt | One Eleven Congress
Extinguishment of Debt [Line Items]
Amount repaid in full				$ 128,000,000
Mortgage Debt | San Jacinto Center
Extinguishment of Debt [Line Items]
Amount repaid in full				$ 101,000,000
Mortgage Debt | Two Buckhead Plaza
Extinguishment of Debt [Line Items]
Amount repaid in full			$ 52,000,000
Mortgage Debt | 3344 Peachtree
Extinguishment of Debt [Line Items]
Amount repaid in full	$ 77,900,000
Mortgage Debt | ACS Center
Extinguishment of Debt [Line Items]
Amount repaid in full		$ 127,000,000
Gain (loss) on extinguishment of debt		$ (376,000)