Notes Receivable, Net - Schedule of Notes Receivable (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) debtinstrument	Dec. 31, 2017 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 109,410	$ 153,829
Number of instruments held | debtinstrument	5
Core Portfolio
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 56,475	101,695
Number of instruments held | debtinstrument	2
Core Portfolio | Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Maturity Date	2019-04
Effective interest rate	6.00%
Core Portfolio | Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Maturity Date	2020-04
Effective interest rate	8.10%
Fund Portfolio | Fund II
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 32,379	31,778
Number of instruments held | debtinstrument	1
Maturity Date	2020-05
Effective interest rate	2.50%
Fund Portfolio | Fund III
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 5,306	5,106
Number of instruments held | debtinstrument	1
Maturity Date	2020-07
Effective interest rate	18.00%
Fund Portfolio | Fund IV
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 15,250	$ 15,250
Number of instruments held | debtinstrument	1
Maturity Date	2021-02
Effective interest rate	15.30%