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Investments - Gross Unrealized Losses and Fair Value for Mortgage-Backed in Unrealized Loss Position (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Less than 12 months |  |
Gross Unrealized Losses | $ (2,876) | $ (2,752)
Estimated Fair Value | 162,036 | 142,116
12 months or more |  |
Gross Unrealized Losses | (151,937) | (150,916)
Estimated Fair Value | 1,723,608 | 2,123,439
Total |  |
Gross Unrealized Losses | (154,813) | (153,668)
Estimated Fair Value | 1,885,644 | 2,265,555
Mortgage-backed securities |  |
Less than 12 months |  |
Gross Unrealized Losses | (1,650) | (531)
Estimated Fair Value | 98,678 | 51,391
12 months or more |  |
Gross Unrealized Losses | (69,521) | (61,949)
Estimated Fair Value | 286,434 | 300,318
Total |  |
Gross Unrealized Losses | (71,171) | (62,480)
Estimated Fair Value | 385,112 | 351,709
Utah Housing Corporation bonds |  |
Less than 12 months |  |
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 0 | 0
12 months or more |  |
Gross Unrealized Losses | (292) | (279)
Estimated Fair Value | 2,909 | 3,129
Total |  |
Gross Unrealized Losses | (292) | (279)
Estimated Fair Value | 2,909 | 3,129
U.S. government-sponsored enterprises and Treasuries |  |
Less than 12 months |  |
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 0 | 0
12 months or more |  |
Gross Unrealized Losses | (63,613) | (66,870)
Estimated Fair Value | 1,232,727 | 1,578,739
Total |  |
Gross Unrealized Losses | (63,613) | (66,870)
Estimated Fair Value | 1,232,727 | 1,578,739
Other securities |  |
Less than 12 months |  |
Gross Unrealized Losses | (1,226) | (2,221)
Estimated Fair Value | 63,358 | 90,725
12 months or more |  |
Gross Unrealized Losses | (18,511) | (21,818)
Estimated Fair Value | 201,538 | 241,253
Total |  |
Gross Unrealized Losses | (19,737) | (24,039)
Estimated Fair Value | $ 264,896 | $ 331,978

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