Investments in Preferred Stock - Schedule of Changes in Level 3 Investments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchase of preferred stock	$ 1,161	$ 2,308
Dividend income from investments in preferred stock	12,303	$ 3,676
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	129,545
Purchase of preferred stock	766
Dividend income from investments in preferred stock	12,303
Unrealized appreciation in the fair value of the investments in preferred stock	5,885
Ending balance	$ 148,499