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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
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Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | $ 16,265,743 | $ 14,932,286
Available-for-sale fixed maturity securities, gross unrealized holding gains | 239,853 | 101,335
Available-for-sale fixed maturity securities, gross unrealized holding losses | (437,748) | (660,889)
Available-for-sale fixed maturity securities, estimated fair value | 16,067,848 | 14,372,732
Short-term investments, available-for-sale, amortized cost | 2,351,758 | 2,564,620
Short-term investments, available-for-sale, gross unrealized holding gains | 4,070 | 7,155
Short-term investments, available-for-sale, gross unrealized holding losses | (90) | (393)
Short-term investments, available-for-sale, estimated fair value | 2,355,738 | 2,571,382
Investments, available-for-sale, amortized cost | 18,617,501 | 17,496,906
Investments, available-for-sale, gross unrealized holding gains | 243,923 | 108,490
Investments, available-for-sale, gross unrealized holding losses | (437,838) | (661,282)
Investments, available-for-sale, estimated fair value | 18,423,586 | 16,944,114
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 4,946,495 | 3,811,293
Available-for-sale fixed maturity securities, gross unrealized holding gains | 79,659 | 35,824
Available-for-sale fixed maturity securities, gross unrealized holding losses | (31,894) | (62,404)
Available-for-sale fixed maturity securities, estimated fair value | 4,994,260 | 3,784,713
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 1,420,858 | 1,225,426
Available-for-sale fixed maturity securities, gross unrealized holding gains | 19,757 | 7,292
Available-for-sale fixed maturity securities, gross unrealized holding losses | (65,452) | (89,904)
Available-for-sale fixed maturity securities, estimated fair value | 1,375,163 | 1,142,814
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 4,003,008 | 4,196,096
Available-for-sale fixed maturity securities, gross unrealized holding gains | 36,672 | 14,787
Available-for-sale fixed maturity securities, gross unrealized holding losses | (129,328) | (181,578)
Available-for-sale fixed maturity securities, estimated fair value | 3,910,352 | 4,029,305
Foreign governments, agencies and supranationals [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 2,783,826 | 2,554,874
Available-for-sale fixed maturity securities, gross unrealized holding gains | 77,781 | 34,352
Available-for-sale fixed maturity securities, gross unrealized holding losses | (103,644) | (142,008)
Available-for-sale fixed maturity securities, estimated fair value | 2,757,963 | 2,447,218
Agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 2,738,131 | 2,507,682
Available-for-sale fixed maturity securities, gross unrealized holding gains | 25,754 | 8,939
Available-for-sale fixed maturity securities, gross unrealized holding losses | (86,205) | (135,611)
Available-for-sale fixed maturity securities, estimated fair value | 2,677,680 | 2,381,010
Non-agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 122,640 | 355,673
Available-for-sale fixed maturity securities, gross unrealized holding gains | 0 | 0
Available-for-sale fixed maturity securities, gross unrealized holding losses | (3,416) | (22,603)
Available-for-sale fixed maturity securities, estimated fair value | 119,224 | 333,070
Corporate and university bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 250,785 | 281,242
Available-for-sale fixed maturity securities, gross unrealized holding gains | 230 | 141
Available-for-sale fixed maturity securities, gross unrealized holding losses | (17,809) | (26,781)
Available-for-sale fixed maturity securities, estimated fair value | $ 233,206 | $ 254,602

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