Revenues - Summary of Amount Receivable from Customers under Various Fee Agreement (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Disaggregation Of Revenue [Line Items]
Balance as of December 31, 2017	$ 17,082
Balance as of September 30, 2018	16,205
Revenue from Contract with Customers Under Various Fee Agreements [Member]
Disaggregation Of Revenue [Line Items]
Balance as of December 31, 2017	1,558
Balance as of September 30, 2018	1,882
Increase	324
Revenue from Contract with Customers Under Various Fee Agreements [Member] | Property Management Fees [Member]
Disaggregation Of Revenue [Line Items]
Balance as of December 31, 2017	290
Balance as of September 30, 2018	472
Increase	182
Revenue from Contract with Customers Under Various Fee Agreements [Member] | Asset Management Fees [Member]
Disaggregation Of Revenue [Line Items]
Balance as of December 31, 2017	762
Balance as of September 30, 2018	894
Increase	132
Revenue from Contract with Customers Under Various Fee Agreements [Member] | Acquisition, Disposition and Leasing Fee [Member]
Disaggregation Of Revenue [Line Items]
Balance as of December 31, 2017	490
Balance as of September 30, 2018	490
Revenue from Contract with Customers Under Various Fee Agreements [Member] | Other Fee Income [Member]
Disaggregation Of Revenue [Line Items]
Balance as of December 31, 2017	16
Balance as of September 30, 2018	26
Increase	$ 10