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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | $ 14,067,742 | $ 12,805,887
Available-for-sale fixed maturity investments, gross unrealized holding gains | 4,275 | 13,100
Available-for-sale fixed maturity investments, gross unrealized holding losses | (1,120,738) | (962,152)
Available-for-sale fixed maturity investments, estimated fair value | 12,951,279 | 11,856,835
Short-term investments, available-for-sale, amortized cost | 2,250,219 | 2,663,560
Short-term investments, available-for-sale, gross unrealized holding gains | 3,253 | 5,760
Short-term investments, available-for-sale, gross unrealized holding losses | (746) | (58)
Short-term investments, available-for-sale, estimated fair value | 2,252,726 | 2,669,262
Investments, available-for-sale, amortized cost | 16,317,961 | 15,469,447
Investments, available-for-sale, gross unrealized holding gains | 7,528 | 18,860
Investments, available-for-sale, gross unrealized holding losses | (1,121,484) | (962,210)
Investments, available-for-sale, estimated fair value | 15,204,005 | 14,526,097
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 3,440,991 | 3,050,089
Available-for-sale fixed maturity investments, gross unrealized holding gains | 0 | 2,363
Available-for-sale fixed maturity investments, gross unrealized holding losses | (130,162) | (138,493)
Available-for-sale fixed maturity investments, estimated fair value | 3,310,829 | 2,913,959
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 1,133,417 | 871,463
Available-for-sale fixed maturity investments, gross unrealized holding gains | 0 | 154
Available-for-sale fixed maturity investments, gross unrealized holding losses | (137,733) | (106,079)
Available-for-sale fixed maturity investments, estimated fair value | 995,684 | 765,538
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 4,106,251 | 3,973,911
Available-for-sale fixed maturity investments, gross unrealized holding gains | 879 | 6,503
Available-for-sale fixed maturity investments, gross unrealized holding losses | (308,184) | (247,231)
Available-for-sale fixed maturity investments, estimated fair value | 3,798,946 | 3,733,183
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 1,749,621 | 1,473,658
Available-for-sale fixed maturity investments, gross unrealized holding gains | 2,626 | 2,843
Available-for-sale fixed maturity investments, gross unrealized holding losses | (189,128) | (169,723)
Available-for-sale fixed maturity investments, estimated fair value | 1,563,119 | 1,306,778
Commercial mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 2,297,828 | 2,109,721
Available-for-sale fixed maturity investments, gross unrealized holding gains | 11 | 395
Available-for-sale fixed maturity investments, gross unrealized holding losses | (208,466) | (169,668)
Available-for-sale fixed maturity investments, estimated fair value | 2,089,373 | 1,940,448
Residential mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 511,525 | 553,591
Available-for-sale fixed maturity investments, gross unrealized holding gains | 1 | 6
Available-for-sale fixed maturity investments, gross unrealized holding losses | (32,686) | (26,804)
Available-for-sale fixed maturity investments, estimated fair value | 478,840 | 526,793
Asset-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 1,226 | 1,693
Available-for-sale fixed maturity investments, gross unrealized holding gains | 0 | 0
Available-for-sale fixed maturity investments, gross unrealized holding losses | (28) | (53)
Available-for-sale fixed maturity investments, estimated fair value | 1,198 | 1,640
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale, amortized cost | 826,883 | 771,761
Available-for-sale fixed maturity investments, gross unrealized holding gains | 758 | 836
Available-for-sale fixed maturity investments, gross unrealized holding losses | (114,351) | (104,101)
Available-for-sale fixed maturity investments, estimated fair value | $ 713,290 | $ 668,496

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