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Investments - Gross Unrealized Losses and Fair Value for Mortgage-Backed in Unrealized Loss Position (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Less than 12 months |  |
Gross Unrealized Losses | $ (5,393) | $ (58,293)
Estimated Fair Value | 148,846 | 1,024,671
12 months or more |  |
Gross Unrealized Losses | (203,858) | (153,985)
Estimated Fair Value | 2,095,461 | 1,313,469
Total |  |
Gross Unrealized Losses | (209,251) | (212,278)
Estimated Fair Value | 2,244,307 | 2,338,140
Mortgage-backed securities |  |
Less than 12 months |  |
Gross Unrealized Losses | (3,342) | (13,956)
Estimated Fair Value | 67,737 | 99,598
12 months or more |  |
Gross Unrealized Losses | (84,057) | (54,749)
Estimated Fair Value | 283,391 | 220,576
Total |  |
Gross Unrealized Losses | (87,399) | (68,705)
Estimated Fair Value | 351,128 | 320,174
Utah Housing Corporation bonds |  |
Less than 12 months |  |
Gross Unrealized Losses | 0 | (357)
Estimated Fair Value | 0 | 3,227
12 months or more |  |
Gross Unrealized Losses | (555) | 0
Estimated Fair Value | 2,853 | 0
Total |  |
Gross Unrealized Losses | (555) | (357)
Estimated Fair Value | 2,853 | 3,227
U.S. government-sponsored enterprises and Treasuries |  |
Less than 12 months |  |
Gross Unrealized Losses | 0 | (28,128)
Estimated Fair Value | 0 | 689,300
12 months or more |  |
Gross Unrealized Losses | (96,757) | (87,288)
Estimated Fair Value | 1,572,991 | 1,000,010
Total |  |
Gross Unrealized Losses | (96,757) | (115,416)
Estimated Fair Value | 1,572,991 | 1,689,310
Other securities |  |
Less than 12 months |  |
Gross Unrealized Losses | (2,051) | (15,852)
Estimated Fair Value | 81,109 | 232,546
12 months or more |  |
Gross Unrealized Losses | (22,489) | (11,948)
Estimated Fair Value | 236,226 | 92,883
Total |  |
Gross Unrealized Losses | (24,540) | (27,800)
Estimated Fair Value | $ 317,335 | $ 325,429

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