Transactions with Norfolk Southern Railway Company (Details) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended		30 Months Ended
	Mar. 01, 2019	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2021	Dec. 31, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets		$ 6,945,970,000		$ 6,945,970,000				$ 6,945,970,000	$ 7,107,398,000
Deferred income		78,331,000		78,331,000				78,331,000	62,319,000
NS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contract price that is expected to be recognized as revenue	$ 52,300,000
Revenue recognized		4,200,000	$ 3,700,000	7,900,000	$ 7,500,000
Deferred income		$ 2,700,000		$ 2,700,000				2,700,000	$ 5,700,000
Contract with customer, liability, revenue recognized								$ 44,200,000
1200 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets	92,300,000
Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property purchased						$ 6,500,000	$ 37,000,000
Site Preparation Work
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property purchased	4,000,000
NS | 1200 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase of a building	$ 82,000,000
Lease term	3 years
Discount on service contract price	$ 10,300,000
Gain or loss on derecognition of non financial asset fair value	0
Consideration for the various services component	52,300,000
NS | Fees for development services
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contract fees	5,000,000
NS | Fees for consulting services
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contract fees	32,000,000
NS | Development agreement | 1200 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration for various services contract	5,000,000
NS | Consulting agreement | 1200 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration for various services contract	32,000,000
NS | Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of property	52,500,000
Gain from sale of property	5,000,000
NS | Land | 1200 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration for various services contract	$ 5,000,000