Transactions with Norfolk Southern Railway Company (Additional Information) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 01, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of property				$ 57,772	$ 0
Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property purchased			$ 6,500			$ 37,000
Site Preparation Work
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property purchased			$ 4,000
NS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue recognized		$ 5,000		$ 11,600
NS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Transaction price	$ 52,300
NS | 1200 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase of a building	$ 82,000
Lease term	3 years
Impairment of property	$ 10,300
NS | Fees for development services
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contract fees	5,000
NS | Fees for consulting services
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contract fees	32,000
NS | Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of property	52,500
Gain from sale of property	$ 5,000