Notes Receivable, Net - Schedule of Notes Receivable (Details) $ in Thousands	Dec. 31, 2017 USD ($) debtinstrument	Dec. 31, 2016 USD ($)
Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 153,829	$ 276,163
Number of instruments held | debtinstrument	6
Core Portfolio
Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 101,695	216,400
Number of instruments held | debtinstrument	3
Core Portfolio | Minimum
Mortgage Loans on Real Estate [Line Items]
Effective interest rate	6.00%
Core Portfolio | Maximum
Mortgage Loans on Real Estate [Line Items]
Effective interest rate	8.10%
Fund Portfolio | Fund II
Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 31,778	31,007
Number of instruments held | debtinstrument	1
Effective interest rate	2.50%
Fund Portfolio | Fund III
Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 5,106	4,506
Number of instruments held | debtinstrument	1
Effective interest rate	18.00%
Fund Portfolio | Fund IV
Mortgage Loans on Real Estate [Line Items]
Net carrying amount of notes receivable | $	$ 15,250	$ 24,250
Number of instruments held | debtinstrument	1
Effective interest rate	15.30%