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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
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Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | $ 15,227,752 | $ 14,932,286
Available-for-sale fixed maturity investments, gross unrealized holding gains | 40,835 | 101,335
Available-for-sale fixed maturity investments, gross unrealized holding losses | (749,217) | (660,889)
Available-for-sale fixed maturity investments, estimated fair value | 14,519,370 | 14,372,732
Short-term investments, available-for-sale, amortized cost | 2,618,162 | 2,564,620
Short-term investments, available-for-sale, gross unrealized holding gains | 597 | 7,155
Short-term investments, available-for-sale, gross unrealized holding losses | (1,117) | (393)
Short-term investments, available-for-sale, estimated fair value | 2,617,642 | 2,571,382
Investments, available-for-sale, amortized cost | 17,845,914 | 17,496,906
Investments, available-for-sale, gross unrealized holding gains | 41,432 | 108,490
Investments, available-for-sale, gross unrealized holding losses | (750,334) | (661,282)
Investments, available-for-sale, estimated fair value | 17,137,012 | 16,944,114
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 4,113,319 | 3,811,293
Available-for-sale fixed maturity investments, gross unrealized holding gains | 12,079 | 35,824
Available-for-sale fixed maturity investments, gross unrealized holding losses | (79,408) | (62,404)
Available-for-sale fixed maturity investments, estimated fair value | 4,045,990 | 3,784,713
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 1,289,119 | 1,225,426
Available-for-sale fixed maturity investments, gross unrealized holding gains | 4,206 | 7,292
Available-for-sale fixed maturity investments, gross unrealized holding losses | (99,430) | (89,904)
Available-for-sale fixed maturity investments, estimated fair value | 1,193,895 | 1,142,814
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 4,041,845 | 4,196,096
Available-for-sale fixed maturity investments, gross unrealized holding gains | 10,059 | 14,787
Available-for-sale fixed maturity investments, gross unrealized holding losses | (205,316) | (181,578)
Available-for-sale fixed maturity investments, estimated fair value | 3,846,588 | 4,029,305
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 1,950,839 | 1,858,845
Available-for-sale fixed maturity investments, gross unrealized holding gains | 4,132 | 21,450
Available-for-sale fixed maturity investments, gross unrealized holding losses | (126,930) | (96,874)
Available-for-sale fixed maturity investments, estimated fair value | 1,828,041 | 1,783,421
Commercial mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 2,400,848 | 2,371,406
Available-for-sale fixed maturity investments, gross unrealized holding gains | 4,780 | 8,605
Available-for-sale fixed maturity investments, gross unrealized holding losses | (137,472) | (136,353)
Available-for-sale fixed maturity investments, estimated fair value | 2,268,156 | 2,243,658
Residential mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 463,710 | 491,949
Available-for-sale fixed maturity investments, gross unrealized holding gains | 155 | 334
Available-for-sale fixed maturity investments, gross unrealized holding losses | (22,187) | (21,861)
Available-for-sale fixed maturity investments, estimated fair value | 441,678 | 470,422
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 968,072 | 977,271
Available-for-sale fixed maturity investments, gross unrealized holding gains | 5,424 | 13,043
Available-for-sale fixed maturity investments, gross unrealized holding losses | (78,474) | (71,915)
Available-for-sale fixed maturity investments, estimated fair value | $ 895,022 | $ 918,399

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