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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 2,013,021 | $ 485,756
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 10,129 | 2,675
Gross Unrealized Losses | (2,202) | (762)
Estimated Fair Value | 2,020,948 | 487,669
Mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 312,549 | 215,888
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 5,832 | 1,895
Gross Unrealized Losses | (378) | (658)
Estimated Fair Value | 318,003 | 217,125
Utah Housing Corporation bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 15,931 | 19,474
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 224 | 145
Gross Unrealized Losses | 0 | (83)
Estimated Fair Value | 16,155 | 19,536
U.S. government-sponsored enterprises and Treasuries |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,608,837 | 250,394
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 4,073 | 635
Gross Unrealized Losses | (6) | (21)
Estimated Fair Value | 1,612,904 | $ 251,008
Other securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 75,704 |
Allowance for credit losses | 0 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (1,818) |
Estimated Fair Value | $ 73,886 |

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