Investment Securities, at Fair Value - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Investment securities, at fair value	$ 40,974,000	$ 47,215,000
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities owned | security	8	8
Investment securities, at fair value	$ 40,974,000	$ 47,215,000
Number of securities with fair value below amortized cost | security	6
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%
Other Income [Member] | CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Gain on sale of investment securities	$ 0	$ 0	$ 100,000