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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 2,423,183 | $ 2,563,984
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 2,738 | 1,306
Gross Unrealized Losses | (154,813) | (153,668)
Estimated Fair Value | 2,271,108 | 2,411,622
Mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 485,026 | 468,204
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 353 | 703
Gross Unrealized Losses | (71,171) | (62,480)
Estimated Fair Value | 414,208 | 406,427
Utah Housing Corporation bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3,201 | 3,408
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (292) | (279)
Estimated Fair Value | 2,909 | 3,129
U.S. government-sponsored enterprises and Treasuries |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,396,341 | 1,645,609
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (63,613) | (66,870)
Estimated Fair Value | 1,332,728 | 1,578,739
Other securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 538,615 | 446,763
Allowance for credit losses | 0 | 0
Gross Unrealized Gains | 2,385 | 603
Gross Unrealized Losses | (19,737) | (24,039)
Estimated Fair Value | $ 521,263 | $ 423,327

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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://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
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