Agreements and Transactions with Related Parties - Due from Affiliates (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Due from affiliates
Short-term loans to affiliates, including accrued interest	$ 57,502	$ 84,031
Deferred acquisition fees receivable, including accrued interest	9,025	12,345
Accounts receivable	4,507	4,089
Reimbursable costs	3,860	4,315
Asset management fees receivable	480	356
Current acquisition fees receivable	166	83
Organization and offering costs	0	89
Due from affiliates	$ 75,540	$ 105,308