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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023
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Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | $ 16,457,723 |
Available-for-sale fixed maturity investments, estimated fair value | 15,745,539 | $ 14,372,732
Short-term investments, available-for-sale, amortized cost basis | 2,530,941 | 2,564,620
Short-term investments, available-for-sale, accumulated gross unrealized gain before tax | 548 | 7,155
Short-term investments, available-for-sale, accumulated gross unrealized loss before tax | (6,579) | (393)
Short-term investments, available-for-sale | 2,524,910 | 2,571,382
Investments, available-for-sale, amortized cost basis | 18,988,664 | 17,496,906
Investments, available-for-sale, accumulated gross unrealized gain before tax | 24,788 | 108,490
Investments, available-for-sale, accumulated gross unrealized loss before tax | (743,003) | (661,282)
Investments, available-for-sale | 18,270,449 | 16,944,114
Investments, Available-for-sale, Allowance for Credit Loss | 0 | 0
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 5,147,365 | 3,811,293
Available-for-sale fixed maturity investments, gross unrealized holding gains | 8,962 | 35,824
Available-for-sale fixed maturity investments, gross unrealized holding losses | (68,469) | (62,404)
Available-for-sale fixed maturity investments, estimated fair value | 5,087,858 | 3,784,713
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 1,445,171 | 1,225,426
Available-for-sale fixed maturity investments, gross unrealized holding gains | 2,976 | 7,292
Available-for-sale fixed maturity investments, gross unrealized holding losses | (101,911) | (89,904)
Available-for-sale fixed maturity investments, estimated fair value | 1,346,236 | 1,142,814
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 3,813,146 | 4,196,096
Available-for-sale fixed maturity investments, gross unrealized holding gains | 5,866 | 14,787
Available-for-sale fixed maturity investments, gross unrealized holding losses | (199,520) | (181,578)
Available-for-sale fixed maturity investments, estimated fair value | 3,619,492 | 4,029,305
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 2,909,561 | 2,554,874
Available-for-sale fixed maturity investments, gross unrealized holding gains | 4,264 | 34,352
Available-for-sale fixed maturity investments, gross unrealized holding losses | (207,302) | (142,008)
Available-for-sale fixed maturity investments, estimated fair value | 2,706,523 | 2,447,218
Agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 2,771,589 | 2,507,682
Available-for-sale fixed maturity investments, gross unrealized holding gains | 2,096 | 8,939
Available-for-sale fixed maturity investments, gross unrealized holding losses | (123,872) | (135,611)
Available-for-sale fixed maturity investments, estimated fair value | 2,649,813 | 2,381,010
Non-agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 122,373 | 355,673
Available-for-sale fixed maturity investments, gross unrealized holding gains | 0 | 0
Available-for-sale fixed maturity investments, gross unrealized holding losses | (4,343) | (22,603)
Available-for-sale fixed maturity investments, estimated fair value | 118,030 | 333,070
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 248,518 | 281,242
Available-for-sale fixed maturity investments, gross unrealized holding gains | 76 | 141
Available-for-sale fixed maturity investments, gross unrealized holding losses | (31,007) | (26,781)
Available-for-sale fixed maturity investments, estimated fair value | 217,587 | 254,602
Total fixed maturities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 16,457,723 | 14,932,286
Available-for-sale fixed maturity investments, gross unrealized holding gains | 24,240 | 101,335
Available-for-sale fixed maturity investments, gross unrealized holding losses | (736,424) | (660,889)
Available-for-sale fixed maturity investments, estimated fair value | $ 15,745,539 | $ 14,372,732

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