Investments in Preferred Stock - Schedule of Changes in Level 3 Investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchase of preferred stock	$ 1,161	$ 0
Dividend income from investments in preferred stock	3,926	$ 0
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	3,926
Unrealized appreciation in the fair value of the investments in preferred stock	424
Ending balance	$ 134,661