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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 2,081,236 | $ 485,756
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 9,924 | 2,675
Gross Unrealized Losses | (453) | (762)
Estimated Fair Value | 2,090,707 | 487,669
Mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 308,757 | 215,888
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 5,670 | 1,895
Gross Unrealized Losses | (100) | (658)
Estimated Fair Value | 314,327 | 217,125
Utah Housing Corporation bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 12,357 | 19,474
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 170 | 145
Gross Unrealized Losses | 0 | (83)
Estimated Fair Value | 12,527 | 19,536
U.S. government-sponsored enterprises and Treasuries |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,687,856 | 250,394
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 3,688 | 635
Gross Unrealized Losses | (10) | (21)
Estimated Fair Value | 1,691,534 | $ 251,008
Other securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 72,266 |
Allowance for credit losses | 0 |
Gross Unrealized Gains | 396 |
Gross Unrealized Losses | (343) |
Estimated Fair Value | $ 72,319 |

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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