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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Jun. 30, 2025 | Mar. 31, 2025 | Dec. 31, 2024 | Jun. 30, 2024 | Mar. 31, 2024 | Dec. 31, 2023
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Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | $ 30,065 |  | $ 44,437 |  |  |
Held-to-maturity, Allowance for Credit Loss | (14) | $ (20) | (25) | $ (34) | $ (38) | $ (43)
Held to maturity, Gross Unrealized Gains | 1,114 |  | 1,561 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 31,165 |  | 45,973 |  |  |
Carrying Value | 30,051 |  | 44,412 |  |  |
Total available for sale, Amortized Cost | 24,339,638 |  | 22,966,462 |  |  |
Available-for-sale, Allowance for Credit Loss | (318) | (333) | (646) | (21,837) | (22,831) | (36,708)
Total available for sale, Gross Unrealized Gains | 245,445 |  | 96,365 |  |  |
Total available for sale, Gross Unrealized Losses | (521,660) |  | (708,728) |  |  |
Total available for sale, Fair Value | 24,063,105 |  | 22,353,453 |  |  |
Total fixed maturity securities available for sale | 24,063,105 |  | 22,353,453 |  |  |
Fixed maturity securities, amortized cost | 24,369,703 |  | 23,010,899 |  |  |
Fixed maturity securities, allowance for credit loss | (332) |  | (671) |  |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 246,559 |  | 97,926 |  |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (521,660) |  | (708,728) |  |  |
Total investments in fixed maturity securities, Fair Value | 24,094,270 |  | 22,399,426 |  |  |
Total investments in fixed maturity securities, Carrying Value | 24,093,156 |  | 22,397,865 |  |  |
State and municipal |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 27,983 |  | 42,145 |  |  |
Held-to-maturity, Allowance for Credit Loss | (14) |  | (25) |  |  |
Held to maturity, Gross Unrealized Gains | 1,040 |  | 1,492 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 29,009 |  | 43,612 |  |  |
Carrying Value | 27,969 |  | 42,120 |  |  |
Total available for sale, Amortized Cost | 2,188,603 |  | 2,376,541 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 | (10) | 0 | (339) | (693) | (757)
Total available for sale, Gross Unrealized Gains | 13,218 |  | 7,560 |  |  |
Total available for sale, Gross Unrealized Losses | (55,220) |  | (89,457) |  |  |
Total available for sale, Fair Value | 2,146,601 |  | 2,294,644 |  |  |
Total fixed maturity securities available for sale | 2,146,601 |  | 2,294,644 |  |  |
Residential mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 2,082 |  | 2,292 |  |  |
Held-to-maturity, Allowance for Credit Loss | 0 |  | 0 |  |  |
Held to maturity, Gross Unrealized Gains | 74 |  | 69 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 2,156 |  | 2,361 |  |  |
Carrying Value | 2,082 |  | 2,292 |  |  |
Total available for sale, Amortized Cost | 4,090,672 |  | 3,411,796 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (5) |  |  |
Total available for sale, Gross Unrealized Gains | 34,670 |  | 11,047 |  |  |
Total available for sale, Gross Unrealized Losses | (157,265) |  | (189,630) |  |  |
Total available for sale, Fair Value | 3,968,077 |  | 3,233,208 |  |  |
Total fixed maturity securities available for sale | 3,968,077 |  | 3,233,208 |  |  |
U.S. government and government agency |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,108,090 |  | 2,268,596 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 23,421 |  | 9,608 |  |  |
Total available for sale, Gross Unrealized Losses | (30,449) |  | (42,863) |  |  |
Total available for sale, Fair Value | 3,101,062 |  | 2,235,341 |  |  |
Total fixed maturity securities available for sale | 3,101,062 |  | 2,235,341 |  |  |
Special revenue |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,387,506 |  | 1,581,778 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 6,306 |  | 3,521 |  |  |
Total available for sale, Gross Unrealized Losses | (40,427) |  | (67,591) |  |  |
Total available for sale, Fair Value | 1,353,385 |  | 1,517,708 |  |  |
Total fixed maturity securities available for sale | 1,353,385 |  | 1,517,708 |  |  |
State general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 258,795 |  | 272,936 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,817 |  | 1,439 |  |  |
Total available for sale, Gross Unrealized Losses | (5,451) |  | (8,981) |  |  |
Total available for sale, Fair Value | 256,161 |  | 265,394 |  |  |
Total fixed maturity securities available for sale | 256,161 |  | 265,394 |  |  |
Pre-refunded |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 77,145 |  | 85,340 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 555 |  | 599 |  |  |
Total available for sale, Gross Unrealized Losses | (223) |  | (347) |  |  |
Total available for sale, Fair Value | 77,477 |  | 85,592 |  |  |
Total fixed maturity securities available for sale | 77,477 |  | 85,592 |  |  |
Corporate backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 187,565 |  | 158,322 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,688 |  | 1,079 |  |  |
Total available for sale, Gross Unrealized Losses | (5,053) |  | (5,827) |  |  |
Total available for sale, Fair Value | 184,200 |  | 153,574 |  |  |
Total fixed maturity securities available for sale | 184,200 |  | 153,574 |  |  |
Local general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 277,592 |  | 278,165 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,852 |  | 922 |  |  |
Total available for sale, Gross Unrealized Losses | (4,066) |  | (6,711) |  |  |
Total available for sale, Fair Value | 275,378 |  | 272,376 |  |  |
Total fixed maturity securities available for sale | 275,378 |  | 272,376 |  |  |
Commercial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 355,922 |  | 534,936 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (425) |  |  |
Total available for sale, Gross Unrealized Gains | 3,911 |  | 1,201 |  |  |
Total available for sale, Gross Unrealized Losses | (333) |  | (3,430) |  |  |
Total available for sale, Fair Value | 359,500 |  | 532,282 |  |  |
Total fixed maturity securities available for sale | 359,500 |  | 532,282 |  |  |
Mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 4,446,594 |  | 3,946,732 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (430) | (1,140) | (562) | (158)
Total available for sale, Gross Unrealized Gains | 38,581 |  | 12,248 |  |  |
Total available for sale, Gross Unrealized Losses | (157,598) |  | (193,060) |  |  |
Total available for sale, Fair Value | 4,327,577 |  | 3,765,490 |  |  |
Total fixed maturity securities available for sale | 4,327,577 |  | 3,765,490 |  |  |
Asset-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,862,993 |  | 3,910,363 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 | (889) | (1,097) | (1,164)
Total available for sale, Gross Unrealized Gains | 17,247 |  | 16,161 |  |  |
Total available for sale, Gross Unrealized Losses | (26,805) |  | (41,512) |  |  |
Total available for sale, Fair Value | 3,853,435 |  | 3,885,012 |  |  |
Total fixed maturity securities available for sale | 3,853,435 |  | 3,885,012 |  |  |
Industrial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,680,399 |  | 3,746,501 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 45,729 |  | 14,518 |  |  |
Total available for sale, Gross Unrealized Losses | (53,582) |  | (93,820) |  |  |
Total available for sale, Fair Value | 3,672,546 |  | 3,667,199 |  |  |
Total fixed maturity securities available for sale | 3,672,546 |  | 3,667,199 |  |  |
Financial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,417,541 |  | 3,339,718 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 54,356 |  | 18,871 |  |  |
Total available for sale, Gross Unrealized Losses | (19,722) |  | (38,076) |  |  |
Total available for sale, Fair Value | 3,452,175 |  | 3,320,513 |  |  |
Total fixed maturity securities available for sale | 3,452,175 |  | 3,320,513 |  |  |
Utilities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,146,714 |  | 795,839 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 15,476 |  | 2,970 |  |  |
Total available for sale, Gross Unrealized Losses | (10,772) |  | (20,115) |  |  |
Total available for sale, Fair Value | 1,151,418 |  | 778,694 |  |  |
Total fixed maturity securities available for sale | 1,151,418 |  | 778,694 |  |  |
Other |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 481,868 |  | 653,194 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,775 |  | 2,493 |  |  |
Total available for sale, Gross Unrealized Losses | (2,006) |  | (4,452) |  |  |
Total available for sale, Fair Value | 482,637 |  | 651,235 |  |  |
Total fixed maturity securities available for sale | 482,637 |  | 651,235 |  |  |
Corporate |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 8,726,522 |  | 8,535,252 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 | 0 |  | (5,026)
Total available for sale, Gross Unrealized Gains | 118,336 |  | 38,852 |  |  |
Total available for sale, Gross Unrealized Losses | (86,082) |  | (156,463) |  |  |
Total available for sale, Fair Value | 8,758,776 |  | 8,417,641 |  |  |
Total fixed maturity securities available for sale | 8,758,776 |  | 8,417,641 |  |  |
Foreign government |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 2,006,836 |  | 1,928,978 |  |  |
Available-for-sale, Allowance for Credit Loss | (318) | $ (323) | (216) | $ (19,469) | $ (20,479) | $ (29,603)
Total available for sale, Gross Unrealized Gains | 34,642 |  | 11,936 |  |  |
Total available for sale, Gross Unrealized Losses | (165,506) |  | (185,373) |  |  |
Total available for sale, Fair Value | 1,875,654 |  | 1,755,325 |  |  |
Total fixed maturity securities available for sale | $ 1,875,654 |  | $ 1,755,325 |  |  |

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