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Investments - Additional Information (Details) $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2020 USD ($) | Sep. 30, 2019 USD ($) | Jun. 30, 2018 USD ($) security | Dec. 31, 2020 USD ($) security numberOfInvestments | Dec. 31, 2019 USD ($) security | Dec. 31, 2018 USD ($)
Investment Holdings [Line Items] |  |  |  |  |  |
Amortized Cost |  |  |  | $ 1,986,957 | $ 485,756 |
Vertical risk retention interest | 500.00% |  |  | 5.00% |  |
Par value of mortgage-backed securities pledged to FRB |  |  |  | $ 815,000 | 252,000 |
Gain on equity securities investment |  | $ 8,000 |  |  |  |
Non-marketable securities investment |  |  |  | 26,000 |  |
Low income housing tax credit investments |  |  |  | 6,000 | 6,000 | $ 4,000
Amount of amortization reported in income tax expense |  |  |  | 5,000 | 4,000 | $ 4,000
Total carrying value of LIHTC investments |  |  |  | 54,000 | 58,000 |
Liability for unfunded commitments |  |  |  | $ 19,000 | 29,000 |
Minimum |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Number of government sponsored securities with unrealized losses | numberOfInvestments |  |  |  | 8 |  |
Maximum |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Number of government sponsored securities with unrealized losses | numberOfInvestments |  |  |  | 10 |  |
Student Loan |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Private education loans sold | $ 1,700,000 |  |  |  |  |
Mortgage-backed securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Amortized Cost |  |  |  | $ 308,913 | $ 215,888 |
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 |  |  |  | 6 | 33 |
Mortgage-backed securities were issued under Ginnie Mae programs that carry a full faith and credit guarantee from the U.S. Government |  |  |  | 32.00% |  |
Number of mortgage-backed securities | security |  |  |  |  | 107 |
Utah Housing Corporation bonds |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Amortized Cost |  |  |  | $ 12,357 | $ 19,474 |
Utah Housing Corporation bonds | Aa3 Rating |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Amortized Cost |  |  |  | 12,000 | 19,000 |
U.S. government-sponsored enterprises and Treasuries |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Amortized Cost |  |  |  | $ 1,596,890 | $ 250,394 |
Number of government sponsored securities with unrealized losses | security |  |  |  | 0 |  |
Number of government sponsored securities | security |  |  |  | 30 |  |
Government National Mortgage Association Certificates and Obligations (GNMA) | Mortgage-backed securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Amortized Cost |  |  |  | $ 32,000 |  |
Number of mortgage-backed securities with unrealized losses | security |  |  |  |  | 18 |
Federal National Mortgage Association Certificates and Obligations (FNMA) | Mortgage-backed securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Amortized Cost |  |  |  | 80,000 |  |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) | Mortgage-backed securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Amortized Cost |  |  |  | $ 197,000 |  |

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