Real Estate Transactions (Dispositions) (Details) ft² in Thousands, $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2019 USD ($) a	Dec. 31, 2017 USD ($) ft²
Atlanta, GA | American Cancer Society Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²		996
Sales Price		$ 166,000
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 | ft²		1,038
Sales Price		$ 208,100
Downtown Atlanta
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | a	8
Sales Price	$ 13,250
Gain on sale of property	$ 13,100