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Leases (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary of income from commercial rents included in real estate revenue:      
Base rent $ 5,711 $ 5,613 $ 5,208
Percentage rent 677 $ 495 $ 652
Future minimum rental income on commercial, communication and right-of-way leases:      
2018 5,715    
2019 5,635    
2020 5,555    
2021 5,309    
2022 5,113    
Thereafter $ 21,679    
Maximum      
Property Subject to or Available for Operating Lease [Line Items]      
Commercial lease agreements, contract terms 60 years