Leases - Ground Leases (Details) ft² in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) Lease property	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) ft² Lease property	Sep. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Number of properties subject to ground leases | property	103		103
Square feet subject to ground leases | ft²			8.7
Number of prepaid ground leases			42
Amortization of prepaid rent | $	$ 0.2	$ 0.2	$ 0.5	$ 0.4
Number of non-prepaid ground leases	61		61
Minimum
Lessee, Lease, Description [Line Items]
Ground lease, initial term	40 years		40 years
Maximum
Lessee, Lease, Description [Line Items]
Ground lease, initial term	99 years		99 years