Leases - Ground Leases (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2019 ft² property Lease	Dec. 31, 2018 ft² renewal_option Lease
Lessee, Lease, Description [Line Items]
Number of properties subject to ground leases		108	107
Square feet subject to ground leases | ft²		8.9	8,800,000
Number of ground leases prepaid		46
Number of ground leases, excluding prepaid leases		62	60
Amortization of prepaid rent | $	$ 0.6
Minimum [Member]
Lessee, Lease, Description [Line Items]
Ground lease, initial term		40 years	50 years
Ground lease, number of renewal options | renewal_option			1
Ground lease, renewal term			75 years
Maximum [Member]
Lessee, Lease, Description [Line Items]
Ground lease, initial term		99 years	75 years
Ground lease, renewal term			100 years
Weighted Average [Member]
Lessee, Lease, Description [Line Items]
Ground lease, initial term			53 years 10 months 24 days