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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022
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Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | $ 14,932,286 | $ 12,805,887
Available-for-sale fixed maturity investments, gross unrealized holding gains | 101,335 | 13,100
Available-for-sale fixed maturity investments, gross unrealized holding losses | (660,889) | (962,152)
Available-for-sale fixed maturity investments, estimated fair value | 14,372,732 | 11,856,835
Short-term investments, available-for-sale, amortized cost basis | 2,564,620 | 2,663,560
Short-term investments, available-for-sale, accumulated gross unrealized gain before tax | 7,155 | 5,760
Short-term investments, available-for-sale, accumulated gross unrealized loss before tax | (393) | (58)
Short-term investments, available-for-sale | 2,571,382 | 2,669,262
Investments, available-for-sale, amortized cost basis | 17,496,906 | 15,469,447
Investments, available-for-sale, accumulated gross unrealized gain before tax | 108,490 | 18,860
Investments, available-for-sale, accumulated gross unrealized loss before tax | (661,282) | (962,210)
Investments, available-for-sale | 16,944,114 | 14,526,097
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 | 3,811,293 | 3,050,089
Available-for-sale fixed maturity investments, gross unrealized holding gains | 35,824 | 2,363
Available-for-sale fixed maturity investments, gross unrealized holding losses | (62,404) | (138,493)
Available-for-sale fixed maturity investments, estimated fair value | 3,784,713 | 2,913,959
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 1,225,426 | 871,463
Available-for-sale fixed maturity investments, gross unrealized holding gains | 7,292 | 154
Available-for-sale fixed maturity investments, gross unrealized holding losses | (89,904) | (106,079)
Available-for-sale fixed maturity investments, estimated fair value | 1,142,814 | 765,538
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 4,196,096 | 3,973,911
Available-for-sale fixed maturity investments, gross unrealized holding gains | 14,787 | 6,503
Available-for-sale fixed maturity investments, gross unrealized holding losses | (181,578) | (247,231)
Available-for-sale fixed maturity investments, estimated fair value | 4,029,305 | 3,733,183
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 1,858,845 | 1,473,658
Available-for-sale fixed maturity investments, gross unrealized holding gains | 21,450 | 2,843
Available-for-sale fixed maturity investments, gross unrealized holding losses | (96,874) | (169,723)
Available-for-sale fixed maturity investments, estimated fair value | 1,783,421 | 1,306,778
Commercial mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 2,371,406 | 2,109,721
Available-for-sale fixed maturity investments, gross unrealized holding gains | 8,605 | 395
Available-for-sale fixed maturity investments, gross unrealized holding losses | (136,353) | (169,668)
Available-for-sale fixed maturity investments, estimated fair value | 2,243,658 | 1,940,448
Residential mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 491,949 | 553,591
Available-for-sale fixed maturity investments, gross unrealized holding gains | 334 | 6
Available-for-sale fixed maturity investments, gross unrealized holding losses | (21,861) | (26,804)
Available-for-sale fixed maturity investments, estimated fair value | 470,422 | 526,793
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, amortized cost | 977,271 | 773,454
Available-for-sale fixed maturity investments, gross unrealized holding gains | 13,043 | 836
Available-for-sale fixed maturity investments, gross unrealized holding losses | (71,915) | (104,154)
Available-for-sale fixed maturity investments, estimated fair value | $ 918,399 | $ 670,136

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