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Leases - Operating leases (Details)
6 Months Ended
Jun. 30, 2019
item
Lease as Lessee  
Number of operating leases 6
Options to extend - operating leases true
Number of ground leases under operating leases 1
Minimum  
Lease as Lessee  
Remaining terms - operating leases 5 years
Maximum  
Lease as Lessee  
Remaining terms - operating leases 7 years