Leases - Ground Leases (Details) ft² in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) property Lease	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) ft² property Lease	Jun. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Number of properties subject to ground leases | property	104		104
Square feet subject to ground leases | ft²			8.7
Number of prepaid ground leases			42
Amortization of prepaid rent | $	$ 0.2	$ 0.2	$ 0.3	$ 0.3
Number of non-prepaid ground leases	62		62
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