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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | $ 16,716,601 |
Available-for-sale fixed maturity securities, estimated fair value | 16,277,629 | $ 15,745,539
Investments, available-for-sale, amortized cost | 18,817,651 | 18,988,664
Investments, available-for-sale, gross unrealized holding gains | 105,889 | 24,788
Investments, available-for-sale, gross unrealized holding losses | (546,878) | (743,003)
Investments, available-for-sale, estimated fair value | 18,376,662 | 18,270,449
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 5,257,869 | 5,147,365
Available-for-sale fixed maturity securities, gross unrealized holding gains | 39,374 | 8,962
Available-for-sale fixed maturity securities, gross unrealized holding losses | (33,562) | (68,469)
Available-for-sale fixed maturity securities, estimated fair value | 5,263,681 | 5,087,858
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 1,495,738 | 1,445,171
Available-for-sale fixed maturity securities, gross unrealized holding gains | 11,133 | 2,976
Available-for-sale fixed maturity securities, gross unrealized holding losses | (77,553) | (101,911)
Available-for-sale fixed maturity securities, estimated fair value | 1,429,318 | 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 | 3,777,894 | 3,813,146
Available-for-sale fixed maturity securities, gross unrealized holding gains | 15,465 | 5,866
Available-for-sale fixed maturity securities, gross unrealized holding losses | (153,987) | (199,520)
Available-for-sale fixed maturity securities, estimated fair value | 3,639,372 | 3,619,492
Foreign governments, agencies and supranationals [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 3,067,330 | 2,909,561
Available-for-sale fixed maturity securities, gross unrealized holding gains | 26,273 | 4,264
Available-for-sale fixed maturity securities, gross unrealized holding losses | (160,800) | (207,302)
Available-for-sale fixed maturity securities, estimated fair value | 2,932,803 | 2,706,523
Agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 2,776,864 | 2,771,589
Available-for-sale fixed maturity securities, gross unrealized holding gains | 13,005 | 2,096
Available-for-sale fixed maturity securities, gross unrealized holding losses | (90,997) | (123,872)
Available-for-sale fixed maturity securities, estimated fair value | 2,698,872 | 2,649,813
Non-agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 104,737 | 122,373
Available-for-sale fixed maturity securities, gross unrealized holding gains | 0 | 0
Available-for-sale fixed maturity securities, gross unrealized holding losses | (2,783) | (4,343)
Available-for-sale fixed maturity securities, estimated fair value | 101,954 | 118,030
Corporate and university bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 236,169 | 248,518
Available-for-sale fixed maturity securities, gross unrealized holding gains | 268 | 76
Available-for-sale fixed maturity securities, gross unrealized holding losses | (24,808) | (31,007)
Available-for-sale fixed maturity securities, estimated fair value | 211,629 | 217,587
Total fixed maturity securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturity securities, available-for-sale, amortized cost | 16,716,601 | 16,457,723
Available-for-sale fixed maturity securities, gross unrealized holding gains | 105,518 | 24,240
Available-for-sale fixed maturity securities, gross unrealized holding losses | (544,490) | (736,424)
Available-for-sale fixed maturity securities, estimated fair value | 16,277,629 | 15,745,539
Short-term investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Short-term investments, available-for-sale, amortized cost | 2,101,050 | 2,530,941
Short-term investments, available-for-sale, gross unrealized holding gains | 371 | 548
Short-term investments, available-for-sale, gross unrealized holding losses | (2,388) | (6,579)
Short-term investments, available-for-sale, estimated fair value | $ 2,099,033 | $ 2,524,910

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