Investment Securities, at Fair Value - Narrative (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Investment securities, at fair value	$ 35,741,000		$ 40,974,000
Proceeds from investments at maturity	$ 1,351,000	$ 0
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities owned | security	6		8
Investment securities, at fair value	$ 35,741,000		$ 40,974,000
Number of securities matured | security	2
Proceeds from investments at maturity	$ 1,000,000
Number of securities with fair value below amortized cost | security	5
Other-than-temporary impairment losses	$ 0
CMBS [Member] | Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Interest rate	5.90%
CMBS [Member] | Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Interest rate	9.00%