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Investments - Additional Information (Details) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2018 USD ($) security | Dec. 31, 2018 USD ($) security | Dec. 31, 2017 USD ($) security
Investment Holdings [Line Items] |  |  |
Amortized Cost |  | $ 182,325 | $ 247,607
Par value of mortgage-backed securities pledged to FRB |  | 147,000 | 218,000
Mortgage-backed securities |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost |  | $ 159,937 | $ 227,607
Number Of Mortgage-Backed Securities Sold | security | 9 |  |
Proceeds from sale of securities | $ 41,000 |  |
Recognized loss from sale of securities | $ 2,000 |  |
Number of mortgage-backed securities with unrealized losses | security |  | 74 | 62
Number of mortgage-backed securities | security |  | 86 | 92
Utah Housing Corporation bonds |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost |  | $ 22,388 | $ 20,000
Ginnie Mae | Mortgage-backed securities |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost |  | $ 64,000 |
Number of mortgage-backed securities with unrealized losses | security |  | 34 | 31
Fannie Mae | Mortgage-backed securities |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost |  | $ 46,000 |
Freddie Mac | Mortgage-backed securities |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost |  | 50,000 |
Aa3 Rating | Utah Housing Corporation bonds |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost |  | $ 22,000 | $ 0

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