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Investments in Preferred Stock - Schedule of Changes in Level 3 Investments (Details) - USD ($) $ in Thousands | 6 Months Ended
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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 |

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