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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | $ 13,653,557 | $ 12,805,887
Available-for-sale fixed maturity investments, gross unrealized holding gains | 7,934 | 13,100
Available-for-sale fixed maturity investments, gross unrealized holding losses | (915,799) | (962,152)
Available-for-sale fixed maturity investments, estimated fair value | 12,745,692 | 11,856,835
Short-term investments, available-for-sale, amortized cost | 2,036,747 | 2,663,560
Short-term investments, available-for-sale, gross unrealized holding gains | 3,840 | 5,760
Short-term investments, available-for-sale, gross unrealized holding losses | (1,526) | (58)
Short-term investments, available-for-sale, estimated fair value | 2,039,061 | 2,669,262
Investments, available-for-sale, amortized cost | 15,690,304 | 15,469,447
Investments, available-for-sale, gross unrealized holding gains | 11,774 | 18,860
Investments, available-for-sale, gross unrealized holding losses | (917,325) | (962,210)
Investments, available-for-sale, estimated fair value | 14,784,753 | 14,526,097
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 3,262,114 | 3,050,089
Available-for-sale fixed maturity investments, gross unrealized holding gains | 280 | 2,363
Available-for-sale fixed maturity investments, gross unrealized holding losses | (133,542) | (138,493)
Available-for-sale fixed maturity investments, estimated fair value | 3,128,852 | 2,913,959
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 1,044,843 | 871,463
Available-for-sale fixed maturity investments, gross unrealized holding gains | 104 | 154
Available-for-sale fixed maturity investments, gross unrealized holding losses | (106,212) | (106,079)
Available-for-sale fixed maturity investments, estimated fair value | 938,735 | 765,538
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 4,093,715 | 3,973,911
Available-for-sale fixed maturity investments, gross unrealized holding gains | 2,728 | 6,503
Available-for-sale fixed maturity investments, gross unrealized holding losses | (220,108) | (247,231)
Available-for-sale fixed maturity investments, estimated fair value | 3,876,335 | 3,733,183
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 1,691,459 | 1,473,658
Available-for-sale fixed maturity investments, gross unrealized holding gains | 3,717 | 2,843
Available-for-sale fixed maturity investments, gross unrealized holding losses | (151,550) | (169,723)
Available-for-sale fixed maturity investments, estimated fair value | 1,543,626 | 1,306,778
Commercial mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 2,251,522 | 2,109,721
Available-for-sale fixed maturity investments, gross unrealized holding gains | 291 | 395
Available-for-sale fixed maturity investments, gross unrealized holding losses | (178,257) | (169,668)
Available-for-sale fixed maturity investments, estimated fair value | 2,073,556 | 1,940,448
Residential mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 508,692 | 553,591
Available-for-sale fixed maturity investments, gross unrealized holding gains | 3 | 6
Available-for-sale fixed maturity investments, gross unrealized holding losses | (28,773) | (26,804)
Available-for-sale fixed maturity investments, estimated fair value | 479,922 | 526,793
Asset-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 1,241 | 1,693
Available-for-sale fixed maturity investments, gross unrealized holding gains | 0 | 0
Available-for-sale fixed maturity investments, gross unrealized holding losses | (37) | (53)
Available-for-sale fixed maturity investments, estimated fair value | 1,204 | 1,640
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 799,971 | 771,761
Available-for-sale fixed maturity investments, gross unrealized holding gains | 811 | 836
Available-for-sale fixed maturity investments, gross unrealized holding losses | (97,320) | (104,101)
Available-for-sale fixed maturity investments, estimated fair value | $ 703,462 | $ 668,496

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