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Fixed Maturity Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - External Credit Rating, Investment Grade - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022
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U.S. government and government agency |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Less Than 12 Months, Fair Value | $ 384,392 | $ 285,391
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses | 6,655 | 10,219
Available-for-sale Securities, 12 Months or Greater, Fair Value | 614,623 | 453,520
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses | 51,014 | 58,939
Available-for-sale Securities, Fair Value | 999,015 | 738,911
Available For Sale Securities, Gross Unrealized Losses | 57,669 | 69,158
State and municipal |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Less Than 12 Months, Fair Value | 264,273 | 1,720,443
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses | 3,013 | 89,272
Available-for-sale Securities, 12 Months or Greater, Fair Value | 1,680,034 | 598,797
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses | 119,449 | 79,320
Available-for-sale Securities, Fair Value | 1,944,307 | 2,319,240
Available For Sale Securities, Gross Unrealized Losses | 122,462 | 168,592
Mortgage- Backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Less Than 12 Months, Fair Value | 278,819 | 1,099,549
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses | 2,025 | 75,430
Available-for-sale Securities, 12 Months or Greater, Fair Value | 1,360,748 | 473,318
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses | 175,131 | 115,528
Available-for-sale Securities, Fair Value | 1,639,567 | 1,572,867
Available For Sale Securities, Gross Unrealized Losses | 177,156 | 190,958
Asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Less Than 12 Months, Fair Value | 413,511 | 1,569,647
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses | 2,070 | 48,390
Available-for-sale Securities, 12 Months or Greater, Fair Value | 2,176,035 | 2,176,638
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses | 71,136 | 103,932
Available-for-sale Securities, Fair Value | 2,589,546 | 3,746,285
Available For Sale Securities, Gross Unrealized Losses | 73,206 | 152,322
Corporate |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Less Than 12 Months, Fair Value | 874,754 | 3,690,856
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses | 11,975 | 150,115
Available-for-sale Securities, 12 Months or Greater, Fair Value | 4,418,309 | 2,349,281
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses | 231,288 | 256,631
Available-for-sale Securities, Fair Value | 5,293,063 | 6,040,137
Available For Sale Securities, Gross Unrealized Losses | 243,263 | 406,746
Foreign government |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Less Than 12 Months, Fair Value | 204,908 | 477,672
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses | 1,758 | 29,815
Available-for-sale Securities, 12 Months or Greater, Fair Value | 794,174 | 711,786
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses | 135,661 | 105,243
Available-for-sale Securities, Fair Value | 999,082 | 1,189,458
Available For Sale Securities, Gross Unrealized Losses | 137,419 | 135,058
Debt Securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Less Than 12 Months, Fair Value | 2,420,657 | 8,843,558
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses | 27,496 | 403,241
Available-for-sale Securities, 12 Months or Greater, Fair Value | 11,043,923 | 6,763,340
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses | 783,679 | 719,593
Available-for-sale Securities, Fair Value | 13,464,580 | 15,606,898
Available For Sale Securities, Gross Unrealized Losses | $ 811,175 | $ 1,122,834

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