Leases - Ground Leases (Details) ft² in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) property Lease	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) ft² property Lease	Jun. 30, 2018 USD ($)	Dec. 31, 2018 ft² property Lease
Lessee, Lease, Description [Line Items]
Number of ground leases where rent has not yet commenced	1		1
Rental income | $	$ 114,351,000	$ 109,566,000	$ 225,046,000	$ 219,795,000
Number of properties subject to ground leases | property	108		108		107
Number of properties held for sale, excluded from ground leases | property	1		1
Square feet subject to ground leases | ft²			9.0		8.8
Number of prepaid ground leases			46
Amortization of prepaid rent | $	$ 200,000	$ 100,000	$ 300,000	$ 300,000
Number of non-prepaid ground leases	62		62		60
Weighted Average Remaining Term
Lessee, Lease, Description [Line Items]
Ground lease, initial term					53 years 10 months 24 days
Minimum
Lessee, Lease, Description [Line Items]
Ground lease, initial term					50 years
Term of lease, including renewals					75 years
Maximum
Lessee, Lease, Description [Line Items]
Ground lease, initial term					75 years
Term of lease, including renewals					100 years
Accounting Standards Update 2016-02 | Weighted Average Remaining Term
Lessee, Lease, Description [Line Items]
Ground lease, initial term	54 years 9 months 18 days		54 years 9 months 18 days
Term of lease, including renewals			70 years 6 months
Accounting Standards Update 2016-02 | Minimum
Lessee, Lease, Description [Line Items]
Ground lease, initial term	40 years		40 years
Accounting Standards Update 2016-02 | Maximum
Lessee, Lease, Description [Line Items]
Ground lease, initial term	99 years		99 years