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Investments - Additional Information (Details) $ in Thousands | Dec. 31, 2015 USD ($) security | Dec. 31, 2014 USD ($) security | Oct. 31, 2013 USD ($)
--- | --- | --- | ---
Investment Holdings [Line Items] |  |  |
Investments at cost | $ 196,402 | $ 167,740 |
Gain on sale of asset-backed security |  |  | $ 63,800
Par value of mortgage-backed securities pledged to FRB | 188,300 | 160,900 |
Mortgage-backed securities |  |  |
Investment Holdings [Line Items] |  |  |
Investments at cost | $ 196,402 | $ 167,740 |
Number of mortgage-backed securities with unrealized losses | security | 35 | 13 |
Number of mortgage-backed securities | security | 74 | 56 |
Ginnie Mae | Mortgage-backed securities |  |  |
Investment Holdings [Line Items] |  |  |
Investments at cost | $ 93,600 |  |
Number of mortgage-backed securities with unrealized losses | security | 14 | 9 |
Number of mortgage-backed securities | security | 35 |  |
Fannie Mae | Mortgage-backed securities |  |  |
Investment Holdings [Line Items] |  |  |
Investments at cost | $ 78,700 |  |
Freddie Mac | Mortgage-backed securities |  |  |
Investment Holdings [Line Items] |  |  |
Investments at cost | $ 24,100 |  |

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