Fair Value of Financial Instruments - Valuation Techniques Used to Measure Investments (Details) $ in Thousands	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Limited liability investments, at fair value	$ 2,035		$ 3,090
Real estate investments	10,662		10,662
Warrant liability	280
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Limited liability investments, at fair value	3,570		4,124
Real estate investments	10,662	$ 10,662	10,662	$ 10,662	$ 10,662	$ 10,662
Warrant liability	$ 280	$ 307	$ 0	$ 0	$ 0	$ 0
Significant Unobservable Inputs (Level 3) | Cap rates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate investments, measurement input	0.075		0.075
Significant Unobservable Inputs (Level 3) | Minimum | Valuation multiples
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Limited liability investments, measurement input	5.0		5.0
Warrant liability, measurement input	6.0
Significant Unobservable Inputs (Level 3) | Maximum | Valuation multiples
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Limited liability investments, measurement input	8.8		8.8