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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023 | Sep. 30, 2023 | Jun. 30, 2023 | Dec. 31, 2022
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Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Held to maturity, Amortized cost | $ 43,959 |  | $ 53,415 |  |  |
Held-to-maturity, Allowance for Credit Loss | (30) | $ (34) | (43) | $ (48) | $ (53) | $ (114)
Held to maturity, Gross Unrealized Gains | 2,053 |  | 3,239 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 45,982 |  | 56,611 |  |  |
Held to maturity, Carrying Value | 43,929 |  | 53,372 |  |  |
Total available for sale, Amortized Cost | 22,959,189 |  | 20,861,830 |  |  |
Available-for-sale, Allowance for Credit Loss | (6,953) | (21,837) | (36,708) | (46,364) | (44,248) | (37,352)
Total available for sale, Gross Unrealized Gains | 255,622 |  | 110,989 |  |  |
Total available for sale, Gross Unrealized Losses | (587,909) |  | (811,175) |  |  |
Total available for sale, Fair Value | 22,619,949 |  | 20,124,936 |  |  |
Total fixed maturity securities available for sale | 22,619,949 |  | 20,124,936 |  |  |
Fixed maturity securities, amortized cost | 23,003,148 |  | 20,915,245 |  |  |
Fixed maturity securities, allowance for credit loss | (6,983) |  | (36,751) |  |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 257,675 |  | 114,228 |  |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (587,909) |  | (811,175) |  |  |
Total investments in fixed maturity securities, Fair Value | 22,665,931 |  | 20,181,547 |  |  |
Total investments in fixed maturity securities, Carrying Value | 22,663,878 |  | 20,178,308 |  |  |
State and municipal |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Held to maturity, Amortized cost | 41,561 |  | 50,547 |  |  |
Held-to-maturity, Allowance for Credit Loss | (30) |  | (43) |  |  |
Held to maturity, Gross Unrealized Gains | 1,944 |  | 3,132 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 43,475 |  | 53,636 |  |  |
Held to maturity, Carrying Value | 41,531 |  | 50,504 |  |  |
Total available for sale, Amortized Cost | 2,567,941 |  | 2,742,922 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 | (339) | (757) | (821) | 0 | 0
Total available for sale, Gross Unrealized Gains | 18,569 |  | 14,719 |  |  |
Total available for sale, Gross Unrealized Losses | (70,208) |  | (122,462) |  |  |
Total available for sale, Fair Value | 2,516,302 |  | 2,634,422 |  |  |
Total fixed maturity securities available for sale | 2,516,302 |  | 2,634,422 |  |  |
Residential mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Held to maturity, Amortized cost | 2,398 |  | 2,868 |  |  |
Held-to-maturity, Allowance for Credit Loss | 0 |  | 0 |  |  |
Held to maturity, Gross Unrealized Gains | 109 |  | 107 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 2,507 |  | 2,975 |  |  |
Held to maturity, Carrying Value | 2,398 |  | 2,868 |  |  |
Total available for sale, Amortized Cost | 2,736,493 |  | 1,773,206 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 31,527 |  | 12,780 |  |  |
Total available for sale, Gross Unrealized Losses | (130,929) |  | (163,844) |  |  |
Total available for sale, Fair Value | 2,637,091 |  | 1,622,142 |  |  |
Total fixed maturity securities available for sale | 2,637,091 |  | 1,622,142 |  |  |
U.S. government and government agency |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 2,083,085 |  | 1,762,997 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 23,974 |  | 11,403 |  |  |
Total available for sale, Gross Unrealized Losses | (36,074) |  | (57,669) |  |  |
Total available for sale, Fair Value | 2,070,985 |  | 1,716,731 |  |  |
Total fixed maturity securities available for sale | 2,070,985 |  | 1,716,731 |  |  |
Special revenue |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,732,996 |  | 1,682,550 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 8,414 |  | 5,651 |  |  |
Total available for sale, Gross Unrealized Losses | (51,652) |  | (82,006) |  |  |
Total available for sale, Fair Value | 1,689,758 |  | 1,606,195 |  |  |
Total fixed maturity securities available for sale | 1,689,758 |  | 1,606,195 |  |  |
State general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 273,232 |  | 394,429 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 4,565 |  | 3,550 |  |  |
Total available for sale, Gross Unrealized Losses | (7,375) |  | (16,405) |  |  |
Total available for sale, Fair Value | 270,422 |  | 381,574 |  |  |
Total fixed maturity securities available for sale | 270,422 |  | 381,574 |  |  |
Pre-refunded |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 85,683 |  | 103,029 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,260 |  | 1,634 |  |  |
Total available for sale, Gross Unrealized Losses | (90) |  | (185) |  |  |
Total available for sale, Fair Value | 86,853 |  | 104,478 |  |  |
Total fixed maturity securities available for sale | 86,853 |  | 104,478 |  |  |
Corporate backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 168,276 |  | 166,873 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (757) |  |  |
Total available for sale, Gross Unrealized Gains | 1,847 |  | 696 |  |  |
Total available for sale, Gross Unrealized Losses | (4,968) |  | (11,973) |  |  |
Total available for sale, Fair Value | 165,155 |  | 154,839 |  |  |
Total fixed maturity securities available for sale | 165,155 |  | 154,839 |  |  |
Local general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 307,754 |  | 396,041 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,483 |  | 3,188 |  |  |
Total available for sale, Gross Unrealized Losses | (6,123) |  | (11,893) |  |  |
Total available for sale, Fair Value | 304,114 |  | 387,336 |  |  |
Total fixed maturity securities available for sale | 304,114 |  | 387,336 |  |  |
Commercial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 600,597 |  | 657,157 |  |  |
Available-for-sale, Allowance for Credit Loss | (52) |  | (158) |  |  |
Total available for sale, Gross Unrealized Gains | 3,254 |  | 626 |  |  |
Total available for sale, Gross Unrealized Losses | (2,918) |  | (13,312) |  |  |
Total available for sale, Fair Value | 600,881 |  | 644,313 |  |  |
Total fixed maturity securities available for sale | 600,881 |  | 644,313 |  |  |
Mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,337,090 |  | 2,430,363 |  |  |
Available-for-sale, Allowance for Credit Loss | (52) | (1,140) | (158) | (2,100) | (885) | (18)
Total available for sale, Gross Unrealized Gains | 34,781 |  | 13,406 |  |  |
Total available for sale, Gross Unrealized Losses | (133,847) |  | (177,156) |  |  |
Total available for sale, Fair Value | 3,237,972 |  | 2,266,455 |  |  |
Total fixed maturity securities available for sale | 3,237,972 |  | 2,266,455 |  |  |
Asset-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 4,118,052 |  | 4,252,883 |  |  |
Available-for-sale, Allowance for Credit Loss | (546) | (889) | (1,164) | (1,502) | (1,444) | 0
Total available for sale, Gross Unrealized Gains | 26,592 |  | 8,527 |  |  |
Total available for sale, Gross Unrealized Losses | (36,777) |  | (73,206) |  |  |
Total available for sale, Fair Value | 4,107,321 |  | 4,187,040 |  |  |
Total fixed maturity securities available for sale | 4,107,321 |  | 4,187,040 |  |  |
Industrial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,980,058 |  | 3,679,219 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (40) |  |  |
Total available for sale, Gross Unrealized Gains | 55,042 |  | 24,312 |  |  |
Total available for sale, Gross Unrealized Losses | (80,692) |  | (143,936) |  |  |
Total available for sale, Fair Value | 3,954,408 |  | 3,559,555 |  |  |
Total fixed maturity securities available for sale | 3,954,408 |  | 3,559,555 |  |  |
Financial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,326,838 |  | 2,838,220 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (4,986) |  |  |
Total available for sale, Gross Unrealized Gains | 54,929 |  | 14,681 |  |  |
Total available for sale, Gross Unrealized Losses | (33,205) |  | (68,681) |  |  |
Total available for sale, Fair Value | 3,348,562 |  | 2,779,234 |  |  |
Total fixed maturity securities available for sale | 3,348,562 |  | 2,779,234 |  |  |
Utilities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 774,956 |  | 701,865 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 13,319 |  | 6,471 |  |  |
Total available for sale, Gross Unrealized Losses | (13,877) |  | (23,412) |  |  |
Total available for sale, Fair Value | 774,398 |  | 684,924 |  |  |
Total fixed maturity securities available for sale | 774,398 |  | 684,924 |  |  |
Other |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 774,274 |  | 635,975 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 4,715 |  | 1,605 |  |  |
Total available for sale, Gross Unrealized Losses | (3,593) |  | (7,234) |  |  |
Total available for sale, Fair Value | 775,396 |  | 630,346 |  |  |
Total fixed maturity securities available for sale | 775,396 |  | 630,346 |  |  |
Corporate |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 8,856,126 |  | 7,855,279 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 | (5,026) | (5,770) | (8,867) | (4,701)
Total available for sale, Gross Unrealized Gains | 128,005 |  | 47,069 |  |  |
Total available for sale, Gross Unrealized Losses | (131,367) |  | (243,263) |  |  |
Total available for sale, Fair Value | 8,852,764 |  | 7,654,059 |  |  |
Total fixed maturity securities available for sale | 8,852,764 |  | 7,654,059 |  |  |
Foreign government |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,996,895 |  | 1,817,386 |  |  |
Available-for-sale, Allowance for Credit Loss | (6,355) | $ (19,469) | (29,603) | $ (36,171) | $ (33,052) | $ (32,633)
Total available for sale, Gross Unrealized Gains | 23,701 |  | 15,865 |  |  |
Total available for sale, Gross Unrealized Losses | (179,636) |  | (137,419) |  |  |
Total available for sale, Fair Value | 1,834,605 |  | 1,666,229 |  |  |
Total fixed maturity securities available for sale | $ 1,834,605 |  | $ 1,666,229 |  |  |

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