Leases - Operating leases (Details)	12 Months Ended
	Dec. 31, 2019 item
Lease as Lessee
Options to extend - operating leases	true
Number of ground leases under operating leases	1
Minimum
Lease as Lessee
Remaining terms - operating leases	4 years
Maximum
Lease as Lessee
Remaining terms - operating leases	7 years