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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Available for sale: |  |
Amortized Cost | $ 1,986,957 | $ 485,756
Gross Unrealized Gains | 10,162 | 2,675
Gross Unrealized Losses | (485) | (762)
Estimated Fair Value | 1,996,634 | 487,669
Mortgage-backed securities |  |
Available for sale: |  |
Amortized Cost | 308,913 | 215,888
Gross Unrealized Gains | 6,095 | 1,895
Gross Unrealized Losses | (134) | (658)
Estimated Fair Value | 314,874 | 217,125
Utah Housing Corporation bonds |  |
Available for sale: |  |
Amortized Cost | 12,357 | 19,474
Gross Unrealized Gains | 210 | 145
Gross Unrealized Losses | 0 | (83)
Estimated Fair Value | 12,567 | 19,536
U.S. government-sponsored enterprises and Treasuries |  |
Available for sale: |  |
Amortized Cost | 1,596,890 | 250,394
Gross Unrealized Gains | 3,395 | 635
Gross Unrealized Losses | 0 | (21)
Estimated Fair Value | 1,600,285 | $ 251,008
Other securities |  |
Available for sale: |  |
Amortized Cost | 68,797 |
Gross Unrealized Gains | 462 |
Gross Unrealized Losses | (351) |
Estimated Fair Value | $ 68,908 |

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