Transactions with Norfolk Southern Railway Company (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended		31 Months Ended
	Mar. 01, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2021	Dec. 31, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets		$ 6,977,364		$ 6,977,364				$ 6,977,364	$ 7,107,398
Deferred income		76,475		76,475				76,475	62,319
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
Revenue recognized		2,500	$ 3,700	10,400	$ 11,200
Deferred income		$ 3,300		$ 3,300				3,300	$ 5,700
Contract with customer, liability, revenue recognized								$ 46,600
1200 Peachtree
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets	92,300
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 | 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
Discount on service contract price	$ 10,300
Gain or loss on derecognition of non financial asset fair value	0
Consideration for the various services component	52,300
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 | 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
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
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
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