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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Jun. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023
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Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | $ 30,331 |  | $ 44,437 |  |  |
Held-to-maturity, Allowance for Credit Loss | (11) | $ (14) | (25) | $ (30) | $ (34) | $ (43)
Held to maturity, Gross Unrealized Gains | 1,145 |  | 1,561 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 31,465 |  | 45,973 |  |  |
Carrying Value | 30,320 |  | 44,412 |  |  |
Total available for sale, Amortized Cost | 25,105,981 |  | 22,966,462 |  |  |
Available-for-sale, Allowance for Credit Loss | (272) | (318) | (646) | (6,953) | (21,837) | (36,708)
Total available for sale, Gross Unrealized Gains | 312,598 |  | 96,365 |  |  |
Total available for sale, Gross Unrealized Losses | (500,678) |  | (708,728) |  |  |
Total available for sale, Fair Value | 24,917,629 |  | 22,353,453 |  |  |
Total fixed maturity securities available for sale | 24,917,629 |  | 22,353,453 |  |  |
Fixed maturity securities, amortized cost | 25,136,312 |  | 23,010,899 |  |  |
Fixed maturity securities, allowance for credit loss | (283) |  | (671) |  |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 313,743 |  | 97,926 |  |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (500,678) |  | (708,728) |  |  |
Total investments in fixed maturity securities, Fair Value | 24,949,094 |  | 22,399,426 |  |  |
Total investments in fixed maturity securities, Carrying Value | 24,947,949 |  | 22,397,865 |  |  |
State and municipal |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 28,378 |  | 42,145 |  |  |
Held-to-maturity, Allowance for Credit Loss | (11) |  | (25) |  |  |
Held to maturity, Gross Unrealized Gains | 1,068 |  | 1,492 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 29,435 |  | 43,612 |  |  |
Carrying Value | 28,367 |  | 42,120 |  |  |
Total available for sale, Amortized Cost | 2,004,227 |  | 2,376,541 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 | 0 | 0 | (339) | (757)
Total available for sale, Gross Unrealized Gains | 14,003 |  | 7,560 |  |  |
Total available for sale, Gross Unrealized Losses | (44,094) |  | (89,457) |  |  |
Total available for sale, Fair Value | 1,974,136 |  | 2,294,644 |  |  |
Total fixed maturity securities available for sale | 1,974,136 |  | 2,294,644 |  |  |
Residential mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 1,953 |  | 2,292 |  |  |
Held-to-maturity, Allowance for Credit Loss | 0 |  | 0 |  |  |
Held to maturity, Gross Unrealized Gains | 77 |  | 69 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 2,030 |  | 2,361 |  |  |
Carrying Value | 1,953 |  | 2,292 |  |  |
Total available for sale, Amortized Cost | 4,723,950 |  | 3,411,796 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (5) |  |  |
Total available for sale, Gross Unrealized Gains | 54,056 |  | 11,047 |  |  |
Total available for sale, Gross Unrealized Losses | (140,189) |  | (189,630) |  |  |
Total available for sale, Fair Value | 4,637,817 |  | 3,233,208 |  |  |
Total fixed maturity securities available for sale | 4,637,817 |  | 3,233,208 |  |  |
U.S. government and government agency |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,453,203 |  | 2,268,596 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 44,713 |  | 9,608 |  |  |
Total available for sale, Gross Unrealized Losses | (19,679) |  | (42,863) |  |  |
Total available for sale, Fair Value | 3,478,237 |  | 2,235,341 |  |  |
Total fixed maturity securities available for sale | 3,478,237 |  | 2,235,341 |  |  |
Special revenue |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,284,059 |  | 1,581,778 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 6,182 |  | 3,521 |  |  |
Total available for sale, Gross Unrealized Losses | (31,536) |  | (67,591) |  |  |
Total available for sale, Fair Value | 1,258,705 |  | 1,517,708 |  |  |
Total fixed maturity securities available for sale | 1,258,705 |  | 1,517,708 |  |  |
State general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 234,610 |  | 272,936 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 3,580 |  | 1,439 |  |  |
Total available for sale, Gross Unrealized Losses | (4,760) |  | (8,981) |  |  |
Total available for sale, Fair Value | 233,430 |  | 265,394 |  |  |
Total fixed maturity securities available for sale | 233,430 |  | 265,394 |  |  |
Pre-refunded |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 74,625 |  | 85,340 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 807 |  | 599 |  |  |
Total available for sale, Gross Unrealized Losses | (113) |  | (347) |  |  |
Total available for sale, Fair Value | 75,319 |  | 85,592 |  |  |
Total fixed maturity securities available for sale | 75,319 |  | 85,592 |  |  |
Corporate backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 183,872 |  | 158,322 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,945 |  | 1,079 |  |  |
Total available for sale, Gross Unrealized Losses | (4,325) |  | (5,827) |  |  |
Total available for sale, Fair Value | 181,492 |  | 153,574 |  |  |
Total fixed maturity securities available for sale | 181,492 |  | 153,574 |  |  |
Local general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 227,061 |  | 278,165 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,489 |  | 922 |  |  |
Total available for sale, Gross Unrealized Losses | (3,360) |  | (6,711) |  |  |
Total available for sale, Fair Value | 225,190 |  | 272,376 |  |  |
Total fixed maturity securities available for sale | 225,190 |  | 272,376 |  |  |
Commercial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 327,655 |  | 534,936 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (425) |  |  |
Total available for sale, Gross Unrealized Gains | 4,123 |  | 1,201 |  |  |
Total available for sale, Gross Unrealized Losses | (149) |  | (3,430) |  |  |
Total available for sale, Fair Value | 331,629 |  | 532,282 |  |  |
Total fixed maturity securities available for sale | 331,629 |  | 532,282 |  |  |
Mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 5,051,605 |  | 3,946,732 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 | (430) | (52) | (1,140) | (158)
Total available for sale, Gross Unrealized Gains | 58,179 |  | 12,248 |  |  |
Total available for sale, Gross Unrealized Losses | (140,338) |  | (193,060) |  |  |
Total available for sale, Fair Value | 4,969,446 |  | 3,765,490 |  |  |
Total fixed maturity securities available for sale | 4,969,446 |  | 3,765,490 |  |  |
Asset-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,829,765 |  | 3,910,363 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 | (546) | (889) | (1,164)
Total available for sale, Gross Unrealized Gains | 21,637 |  | 16,161 |  |  |
Total available for sale, Gross Unrealized Losses | (20,228) |  | (41,512) |  |  |
Total available for sale, Fair Value | 3,831,174 |  | 3,885,012 |  |  |
Total fixed maturity securities available for sale | 3,831,174 |  | 3,885,012 |  |  |
Industrial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,692,392 |  | 3,746,501 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 56,493 |  | 14,518 |  |  |
Total available for sale, Gross Unrealized Losses | (42,887) |  | (93,820) |  |  |
Total available for sale, Fair Value | 3,705,998 |  | 3,667,199 |  |  |
Total fixed maturity securities available for sale | 3,705,998 |  | 3,667,199 |  |  |
Financial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,497,735 |  | 3,339,718 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 64,506 |  | 18,871 |  |  |
Total available for sale, Gross Unrealized Losses | (17,838) |  | (38,076) |  |  |
Total available for sale, Fair Value | 3,544,403 |  | 3,320,513 |  |  |
Total fixed maturity securities available for sale | 3,544,403 |  | 3,320,513 |  |  |
Utilities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,239,824 |  | 795,839 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 21,902 |  | 2,970 |  |  |
Total available for sale, Gross Unrealized Losses | (8,995) |  | (20,115) |  |  |
Total available for sale, Fair Value | 1,252,731 |  | 778,694 |  |  |
Total fixed maturity securities available for sale | 1,252,731 |  | 778,694 |  |  |
Other |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 236,635 |  | 653,194 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,562 |  | 2,493 |  |  |
Total available for sale, Gross Unrealized Losses | (1,557) |  | (4,452) |  |  |
Total available for sale, Fair Value | 237,640 |  | 651,235 |  |  |
Total fixed maturity securities available for sale | 237,640 |  | 651,235 |  |  |
Corporate |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 8,666,586 |  | 8,535,252 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 | 0 |  | (5,026)
Total available for sale, Gross Unrealized Gains | 145,463 |  | 38,852 |  |  |
Total available for sale, Gross Unrealized Losses | (71,277) |  | (156,463) |  |  |
Total available for sale, Fair Value | 8,740,772 |  | 8,417,641 |  |  |
Total fixed maturity securities available for sale | 8,740,772 |  | 8,417,641 |  |  |
Foreign government |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 2,100,595 |  | 1,928,978 |  |  |
Available-for-sale, Allowance for Credit Loss | (272) | $ (318) | (216) | $ (6,355) | $ (19,469) | $ (29,603)
Total available for sale, Gross Unrealized Gains | 28,603 |  | 11,936 |  |  |
Total available for sale, Gross Unrealized Losses | (205,062) |  | (185,373) |  |  |
Total available for sale, Fair Value | 1,923,864 |  | 1,755,325 |  |  |
Total fixed maturity securities available for sale | $ 1,923,864 |  | $ 1,755,325 |  |  |

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