Related Person Transactions - Two North Riverside Plaza Joint Venture Limited Partnership (Details) - Two North Riverside Plaza Joint Venture Limited Partnership		12 Months Ended
	Jul. 20, 2015 option	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Related Party Transaction [Line Items]
Lease term	5 years
Number of renewal options | option	1
Renewal option term	5 years
Tenant improvements		$ 700,000
Operating lease, termination period duration	30 days
Expenses from transactions with related party		800,000	$ 800,000	$ 800,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity
2019		900,000
2020		900,000
Due to related party		$ 0	$ 0