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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Jun. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2022 | Jun. 30, 2022 | Dec. 31, 2021
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Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | $ 52,876 |  | $ 51,410 |  |  |
Held-to-maturity, Allowance for Credit Loss | (48) | $ (53) | (114) | $ (121) | $ (127) | $ (387)
Held to maturity, Gross Unrealized Gains | 1,844 |  | 4,277 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 54,672 |  | 55,573 |  |  |
Total held to maturity, Carrying Value | 52,828 |  | 51,296 |  |  |
Total available for sale, Amortized Cost | 20,179,897 |  | 18,664,073 |  |  |
Available-for-sale, Allowance for Credit Loss | (46,364) | (44,248) | (37,352) | (34,595) | (33,278) | (22,238)
Total available for sale, Gross Unrealized Gains | 16,946 |  | 32,166 |  |  |
Total available for sale, Gross Unrealized Losses | (1,168,002) |  | (1,122,834) |  |  |
Total available for sale, Fair Value | 18,982,477 |  | 17,536,053 |  |  |
Total fixed maturity securities available for sale | 18,982,477 |  | 17,536,053 |  |  |
Total investments in fixed maturity securities, Amortized Value | 20,232,773 |  | 18,715,483 |  |  |
Fixed maturity securities, allowance for credit loss | (46,412) |  | (37,466) |  |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 18,790 |  | 36,443 |  |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (1,168,002) |  | (1,122,834) |  |  |
Total investments in fixed maturity securities, Fair Value | 19,037,149 |  | 17,591,626 |  |  |
Total investments in fixed maturity securities, Carrying Value | 19,035,305 |  | 17,587,349 |  |  |
State and municipal |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 49,847 |  | 47,802 |  |  |
Held-to-maturity, Allowance for Credit Loss | (48) |  | (114) |  |  |
Held to maturity, Gross Unrealized Gains | 1,825 |  | 4,239 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 51,624 |  | 51,927 |  |  |
Total held to maturity, Carrying Value | 49,799 |  | 47,688 |  |  |
Total available for sale, Amortized Cost | 2,849,350 |  | 3,046,335 |  |  |
Available-for-sale, Allowance for Credit Loss | (821) | 0 | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 5,662 |  | 12,355 |  |  |
Total available for sale, Gross Unrealized Losses | (183,193) |  | (168,592) |  |  |
Total available for sale, Fair Value | 2,670,998 |  | 2,890,098 |  |  |
Total fixed maturity securities available for sale | 2,670,998 |  | 2,890,098 |  |  |
Residential mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 3,029 |  | 3,608 |  |  |
Held-to-maturity, Allowance for Credit Loss | 0 |  | 0 |  |  |
Held to maturity, Gross Unrealized Gains | 19 |  | 38 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 3,048 |  | 3,646 |  |  |
Total held to maturity, Carrying Value | 3,029 |  | 3,608 |  |  |
Total available for sale, Amortized Cost | 1,660,790 |  | 1,308,019 |  |  |
Available-for-sale, Allowance for Credit Loss | (24) |  | (18) |  |  |
Total available for sale, Gross Unrealized Gains | 399 |  | 395 |  |  |
Total available for sale, Gross Unrealized Losses | (214,381) |  | (171,595) |  |  |
Total available for sale, Fair Value | 1,446,784 |  | 1,136,801 |  |  |
Total fixed maturity securities available for sale | 1,446,784 |  | 1,136,801 |  |  |
U.S. government and government agency |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,546,889 |  | 960,479 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,077 |  | 937 |  |  |
Total available for sale, Gross Unrealized Losses | (81,767) |  | (69,158) |  |  |
Total available for sale, Fair Value | 1,467,199 |  | 892,258 |  |  |
Total fixed maturity securities available for sale | 1,467,199 |  | 892,258 |  |  |
Special revenue |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,771,219 |  | 1,837,309 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,287 |  | 3,662 |  |  |
Total available for sale, Gross Unrealized Losses | (126,585) |  | (119,474) |  |  |
Total available for sale, Fair Value | 1,646,921 |  | 1,721,497 |  |  |
Total fixed maturity securities available for sale | 1,646,921 |  | 1,721,497 |  |  |
State general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 378,650 |  | 387,709 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,247 |  | 2,651 |  |  |
Total available for sale, Gross Unrealized Losses | (23,727) |  | (21,335) |  |  |
Total available for sale, Fair Value | 356,170 |  | 369,025 |  |  |
Total fixed maturity securities available for sale | 356,170 |  | 369,025 |  |  |
Pre-refunded |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 103,219 |  | 156,106 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 202 |  | 2,741 |  |  |
Total available for sale, Gross Unrealized Losses | (827) |  | (7) |  |  |
Total available for sale, Fair Value | 102,594 |  | 158,840 |  |  |
Total fixed maturity securities available for sale | 102,594 |  | 158,840 |  |  |
Corporate backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 194,976 |  | 210,228 |  |  |
Available-for-sale, Allowance for Credit Loss | (821) |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 159 |  | 334 |  |  |
Total available for sale, Gross Unrealized Losses | (14,341) |  | (10,923) |  |  |
Total available for sale, Fair Value | 179,973 |  | 199,639 |  |  |
Total fixed maturity securities available for sale | 179,973 |  | 199,639 |  |  |
Local general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 401,286 |  | 454,983 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,767 |  | 2,967 |  |  |
Total available for sale, Gross Unrealized Losses | (17,713) |  | (16,853) |  |  |
Total available for sale, Fair Value | 385,340 |  | 441,097 |  |  |
Total fixed maturity securities available for sale | 385,340 |  | 441,097 |  |  |
Commercial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 651,966 |  | 547,757 |  |  |
Available-for-sale, Allowance for Credit Loss | (2,076) |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 11 |  | 215 |  |  |
Total available for sale, Gross Unrealized Losses | (20,389) |  | (19,363) |  |  |
Total available for sale, Fair Value | 629,512 |  | 528,609 |  |  |
Total fixed maturity securities available for sale | 629,512 |  | 528,609 |  |  |
Mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 2,312,756 |  | 1,855,776 |  |  |
Available-for-sale, Allowance for Credit Loss | (2,100) | (885) | (18) | (21) | 0 | 0
Total available for sale, Gross Unrealized Gains | 410 |  | 610 |  |  |
Total available for sale, Gross Unrealized Losses | (234,770) |  | (190,958) |  |  |
Total available for sale, Fair Value | 2,076,296 |  | 1,665,410 |  |  |
Total fixed maturity securities available for sale | 2,076,296 |  | 1,665,410 |  |  |
Asset-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 4,197,235 |  | 4,132,365 |  |  |
Available-for-sale, Allowance for Credit Loss | (1,502) | (1,444) | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 750 |  | 2,730 |  |  |
Total available for sale, Gross Unrealized Losses | (109,025) |  | (152,322) |  |  |
Total available for sale, Fair Value | 4,087,458 |  | 3,982,773 |  |  |
Total fixed maturity securities available for sale | 4,087,458 |  | 3,982,773 |  |  |
Industrial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,567,020 |  | 3,491,645 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (1,704) |  |  |
Total available for sale, Gross Unrealized Gains | 5,026 |  | 4,439 |  |  |
Total available for sale, Gross Unrealized Losses | (227,479) |  | (241,381) |  |  |
Total available for sale, Fair Value | 3,344,567 |  | 3,252,999 |  |  |
Total fixed maturity securities available for sale | 3,344,567 |  | 3,252,999 |  |  |
Financial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 2,813,603 |  | 2,585,247 |  |  |
Available-for-sale, Allowance for Credit Loss | (5,406) |  | (2,997) |  |  |
Total available for sale, Gross Unrealized Gains | 1,362 |  | 5,505 |  |  |
Total available for sale, Gross Unrealized Losses | (130,312) |  | (117,383) |  |  |
Total available for sale, Fair Value | 2,679,247 |  | 2,470,372 |  |  |
Total fixed maturity securities available for sale | 2,679,247 |  | 2,470,372 |  |  |
Utilities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 683,468 |  | 586,066 |  |  |
Available-for-sale, Allowance for Credit Loss | (364) |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 164 |  | 1,307 |  |  |
Total available for sale, Gross Unrealized Losses | (39,991) |  | (36,325) |  |  |
Total available for sale, Fair Value | 643,277 |  | 551,048 |  |  |
Total fixed maturity securities available for sale | 643,277 |  | 551,048 |  |  |
Other |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 593,466 |  | 441,230 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 134 |  | 0 |  |  |
Total available for sale, Gross Unrealized Losses | (11,877) |  | (11,657) |  |  |
Total available for sale, Fair Value | 581,723 |  | 429,573 |  |  |
Total fixed maturity securities available for sale | 581,723 |  | 429,573 |  |  |
Corporate |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 7,657,557 |  | 7,104,188 |  |  |
Available-for-sale, Allowance for Credit Loss | (5,770) | (8,867) | (4,701) | (1,199) | (182) | (16)
Total available for sale, Gross Unrealized Gains | 6,686 |  | 11,251 |  |  |
Total available for sale, Gross Unrealized Losses | (409,659) |  | (406,746) |  |  |
Total available for sale, Fair Value | 7,248,814 |  | 6,703,992 |  |  |
Total fixed maturity securities available for sale | 7,248,814 |  | 6,703,992 |  |  |
Foreign government |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,616,110 |  | 1,564,930 |  |  |
Available-for-sale, Allowance for Credit Loss | (36,171) | $ (33,052) | (32,633) | $ (33,375) | $ (33,096) | $ (22,222)
Total available for sale, Gross Unrealized Gains | 1,361 |  | 4,283 |  |  |
Total available for sale, Gross Unrealized Losses | (149,588) |  | (135,058) |  |  |
Total available for sale, Fair Value | 1,431,712 |  | 1,401,522 |  |  |
Total fixed maturity securities available for sale | $ 1,431,712 |  | $ 1,401,522 |  |  |

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