Real Estate Transactions (Properties Held for Sale) (Details) ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($) ft²
Atlanta, GA | 100 North Point Center East
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²	129
Sales Price | $	$ 22,000
Atlanta, GA | 191 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²	1,225
Sales Price | $	$ 267,500
Atlanta, GA | The Forum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²	220
Sales Price | $	$ 70,000
Atlanta, GA | 200, 333, and 555 North Point Center East
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²		411
Sales Price | $		$ 70,300
Houston | Post Oak Central
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²	1,280
Sales Price | $	$ (1)
Houston | Greenway Plaza
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²	4,348
Sales Price | $	$ (1)
Philadelphia | Two Liberty Place
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²	941
Sales Price | $	$ 219,000
Miami | Lincoln Place
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²	140
Sales Price | $	$ 80,000
Dallas, TX | 2100 Ross
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²		844
Sales Price | $		$ 131,000
Dallas, TX | The Points at Waterview
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²		203
Sales Price | $		$ 26,800
Ft. Worth, TX | 777 Main
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²			980
Sales Price | $			$ 167,000
Birmingham, AL | Lakeshore Park Plaza
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²			197
Sales Price | $			$ 25,000
Birmingham, AL | 600 University Park Place
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²			123
Sales Price | $			$ 19,700
Tallahassee, FL | Mahan Village
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²			147
Sales Price | $			$ 29,500