Revenues - Summary of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Rental income									$ 667,360	$ 628,883	$ 590,161
Tenant reimbursement income									56,950	52,418	44,943
Fee income:
Total fee income									18,629	24,212	16,931
Lease termination income									2,985	2,189	17,010
Other income									13,037	11,265	14,296
Total fee and other income									34,651	37,666	48,237
Total revenues	$ 190,675	$ 192,596	$ 191,419	$ 184,271	$ 180,257	$ 179,770	$ 177,704	$ 181,236	758,961	718,967	683,341
Property Management Fees [Member]
Fee income:
Total fee income									6,163	6,336	5,948
Asset Management Fees [Member]
Fee income:
Total fee income									7,912	8,581	7,754
Acquisition, Disposition and Leasing Fee [Member]
Fee income:
Total fee income									3,160	7,770	2,026
Other Fee Income [Member]
Fee income:
Total fee income									$ 1,394	$ 1,525	$ 1,203