Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Lease revenues									$ 217,251	$ 223,388	$ 220,764
Rental revenues	$ 55,880	$ 57,760	$ 55,932	$ 56,778	$ 57,567	$ 59,125	$ 58,253	$ 57,880	226,350	232,825	230,574
Parking revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									5,608	5,680	5,850
Tenant services
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									$ 3,491	$ 3,757	$ 3,960