Real Estate Transactions (Details) ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($) ft²
Atlanta, GA | American Cancer Society Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet	996
Sales Price | $	$ 166,000
Atlanta, GA | 191 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet		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		220
Sales Price | $		$ 70,000
Atlanta, GA | 100 North Point Center East
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet		129
Sales Price | $		$ 22,000
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			411
Sales Price | $			$ 70,300
Orlando, FL | Bank of America Center, One Orlando Centre, --and Citrus Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet	1,038
Sales Price | $	$ 208,100
Houston, TX | Post Oak Central
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet		1,280
Houston, TX | Greenway Plaza
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet		4,348
Philadelphia, PA | Two Liberty Place
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet		941
Sales Price | $		$ 219,000
Miami, FL | Lincoln Place
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet		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			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			203
Sales Price | $			$ 26,800