Property Transactions (Properties Held for Sale) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($) ft²	Dec. 31, 2013 USD ($) ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating properties, net held for sale	$ 6,421
Notes and accounts receivable	210
Operating properties accumulated depreciation, held for sale	$ 7,072
Document Fiscal Year Focus	2015	2014	2013
Gain (loss) on sale from discontinued operations	$ (551)	$ 19,358	$ 11,489
Summary of sales in discontinued operations
Document Period End Date	Dec. 31, 2015
Other assets accumulated depreciation, held for sale	$ 128
Deferred rents receivable	496
Held for sale, other assets	119
Assets Held-for-sale, Not Part of Disposal Group	7,246
Accounts payable and accrued expenses	140
Deferred income held for sale	200
Other liabilities	1,007
Total liabilities held for sale	$ 1,347
2100 Ross Avenue [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²	129,000
Sales price of property held for sale, discontinued operations	$ 22,000
200, 333, and 555 North Point Center [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²	411,000,000
Sales price of property held for sale, discontinued operations	$ 70,300
The Points at Waterview [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²	203,000,000
Sales price of property held for sale, discontinued operations	$ 26,800
2100 Ross Avenue [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²	844,000,000
Sales price of property held for sale, discontinued operations	$ 131,000
777 Main Fort Worth, TX [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²		980,000
Sales price of property held for sale, discontinued operations		$ 167,000
Tiffany Springs MarketCenter Kansas City [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²			238,000
Sales price of property held for sale, discontinued operations			$ 53,500
Lakeshore Park Plaza Birmingham [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²		197,000
Sales price of property held for sale, discontinued operations		$ 25,000
Mahan Village Tallahassee FL [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²		147,000
Sales price of property held for sale, discontinued operations		$ 29,500
Six Hundred University Park Place Birmingham [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²		123,000
Sales price of property held for sale, discontinued operations		$ 19,700
Inhibitex Suburban Atlanta, GA [Member]
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet) | ft²			51,000
Sales price of property held for sale, discontinued operations			$ 8,300