Investments in Preferred Stock - Schedule of Changes in Level 3 Investments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchase of preferred stock	$ 1,161	$ 1,167
Dividend income from investments in preferred stock	7,980	$ 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	7,980
Unrealized appreciation in the fair value of the investments in preferred stock	6,451
Ending balance	$ 144,742