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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | $ 17,386,303 |
Available-for-sale fixed maturity securities, estimated fair value | 17,259,981 | $ 15,745,539
Investments, available-for-sale, amortized cost | 19,498,574 | 18,988,664
Investments, available-for-sale, gross unrealized holding gains | 238,265 | 24,788
Investments, available-for-sale, gross unrealized holding losses | (361,101) | (743,003)
Investments, available-for-sale, estimated fair value | 19,375,738 | 18,270,449
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 5,621,479 | 5,147,365
Available-for-sale fixed maturity securities, gross unrealized holding gains | 61,548 | 8,962
Available-for-sale fixed maturity securities, gross unrealized holding losses | (17,504) | (68,469)
Available-for-sale fixed maturity securities, estimated fair value | 5,665,523 | 5,087,858
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 1,604,245 | 1,445,171
Available-for-sale fixed maturity securities, gross unrealized holding gains | 19,744 | 2,976
Available-for-sale fixed maturity securities, gross unrealized holding losses | (61,921) | (101,911)
Available-for-sale fixed maturity securities, estimated fair value | 1,562,068 | 1,346,236
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 3,732,612 | 3,813,146
Available-for-sale fixed maturity securities, gross unrealized holding gains | 34,511 | 5,866
Available-for-sale fixed maturity securities, gross unrealized holding losses | (113,294) | (199,520)
Available-for-sale fixed maturity securities, estimated fair value | 3,653,829 | 3,619,492
Foreign governments, agencies and supranationals [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 3,269,118 | 2,909,561
Available-for-sale fixed maturity securities, gross unrealized holding gains | 92,948 | 4,264
Available-for-sale fixed maturity securities, gross unrealized holding losses | (79,566) | (207,302)
Available-for-sale fixed maturity securities, estimated fair value | 3,282,500 | 2,706,523
Agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 2,866,218 | 2,771,589
Available-for-sale fixed maturity securities, gross unrealized holding gains | 25,412 | 2,096
Available-for-sale fixed maturity securities, gross unrealized holding losses | (67,353) | (123,872)
Available-for-sale fixed maturity securities, estimated fair value | 2,824,277 | 2,649,813
Non-agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 67,746 | 122,373
Available-for-sale fixed maturity securities, gross unrealized holding gains | 2 | 0
Available-for-sale fixed maturity securities, gross unrealized holding losses | (703) | (4,343)
Available-for-sale fixed maturity securities, estimated fair value | 67,045 | 118,030
Corporate and university bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 224,885 | 248,518
Available-for-sale fixed maturity securities, gross unrealized holding gains | 164 | 76
Available-for-sale fixed maturity securities, gross unrealized holding losses | (20,310) | (31,007)
Available-for-sale fixed maturity securities, estimated fair value | 204,739 | 217,587
Total fixed maturity securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 17,386,303 | 16,457,723
Available-for-sale fixed maturity securities, gross unrealized holding gains | 234,329 | 24,240
Available-for-sale fixed maturity securities, gross unrealized holding losses | (360,651) | (736,424)
Available-for-sale fixed maturity securities, estimated fair value | 17,259,981 | 15,745,539
Short-term investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Short-term investments, available-for-sale, amortized cost | 2,112,271 | 2,530,941
Short-term investments, available-for-sale, gross unrealized holding gains | 3,936 | 548
Short-term investments, available-for-sale, gross unrealized holding losses | (450) | (6,579)
Short-term investments, available-for-sale, estimated fair value | $ 2,115,757 | $ 2,524,910

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