Investments in Preferred Stock - Schedule of Changes in Level 3 Investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchase of preferred stock	$ 0	$ 1,161
Dividend income from investments in preferred stock	4,883	$ 3,926
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	161,225
Dividend income from investments in preferred stock	4,883
Net unrealized depreciation in the fair value of the investments in preferred stock	(39)
Ending balance	$ 166,069