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Investments - Additional Information (Details) $ in Thousands | Dec. 31, 2016 USD ($) security | Dec. 31, 2015 USD ($) security
--- | --- | ---
Investment Holdings [Line Items] |  |
Amortized Cost | $ 211,406 | $ 196,402
Par value of mortgage-backed securities pledged to FRB | 188,000 | 188,300
Mortgage-backed securities |  |
Investment Holdings [Line Items] |  |
Amortized Cost | $ 196,406 | $ 196,402
Number of mortgage-backed securities with unrealized losses | security | 48 | 35
Number of mortgage-backed securities | security | 84 | 74
Utah Housing Corporation bonds |  |
Investment Holdings [Line Items] |  |
Amortized Cost | $ 15,000 |
Ginnie Mae | Mortgage-backed securities |  |
Investment Holdings [Line Items] |  |
Amortized Cost | $ 101,200 |
Number of mortgage-backed securities with unrealized losses | security | 21 | 14
Fannie Mae | Mortgage-backed securities |  |
Investment Holdings [Line Items] |  |
Amortized Cost | $ 66,500 |
Freddie Mac | Mortgage-backed securities |  |
Investment Holdings [Line Items] |  |
Amortized Cost | 28,800 |
Aa3 Rating | Utah Housing Corporation bonds |  |
Investment Holdings [Line Items] |  |
Amortized Cost | $ 15,000 |

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