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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Jun. 30, 2023 | Mar. 31, 2023 | Dec. 31, 2022 | Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021
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Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | $ 52,364 |  | $ 51,410 |  |  |
Held-to-maturity, Allowance for Credit Loss | (53) | $ (107) | (114) | $ (127) | $ (378) | $ (387)
Held to maturity, Gross Unrealized Gains | 2,882 |  | 4,277 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 55,193 |  | 55,573 |  |  |
Total held to maturity, Carrying Value | 52,311 |  | 51,296 |  |  |
Total available for sale, Amortized Cost | 19,051,984 |  | 18,664,073 |  |  |
Available-for-sale, Allowance for Credit Loss | (44,248) | (37,142) | (37,352) | (33,278) | (26,153) | (22,238)
Total available for sale, Gross Unrealized Gains | 24,829 |  | 32,166 |  |  |
Total available for sale, Gross Unrealized Losses | (1,029,253) |  | (1,122,834) |  |  |
Total available for sale, Fair Value | 18,003,312 |  | 17,536,053 |  |  |
Total fixed maturity securities available for sale | 18,003,312 |  | 17,536,053 |  |  |
Total investments in fixed maturity securities, Amortized Value | 19,104,348 |  | 18,715,483 |  |  |
Fixed maturity securities, allowance for credit loss | (44,301) |  | (37,466) |  |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 27,711 |  | 36,443 |  |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (1,029,253) |  | (1,122,834) |  |  |
Total investments in fixed maturity securities, Fair Value | 18,058,505 |  | 17,591,626 |  |  |
Total investments in fixed maturity securities, Carrying Value | 18,055,623 |  | 17,587,349 |  |  |
State and municipal |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 49,156 |  | 47,802 |  |  |
Held-to-maturity, Allowance for Credit Loss | (53) |  | (114) |  |  |
Held to maturity, Gross Unrealized Gains | 2,859 |  | 4,239 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 51,962 |  | 51,927 |  |  |
Total held to maturity, Carrying Value | 49,103 |  | 47,688 |  |  |
Total available for sale, Amortized Cost | 2,837,668 |  | 3,046,335 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 10,371 |  | 12,355 |  |  |
Total available for sale, Gross Unrealized Losses | (144,995) |  | (168,592) |  |  |
Total available for sale, Fair Value | 2,703,044 |  | 2,890,098 |  |  |
Total fixed maturity securities available for sale | 2,703,044 |  | 2,890,098 |  |  |
Residential mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 3,208 |  | 3,608 |  |  |
Held-to-maturity, Allowance for Credit Loss | 0 |  | 0 |  |  |
Held to maturity, Gross Unrealized Gains | 23 |  | 38 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 3,231 |  | 3,646 |  |  |
Total held to maturity, Carrying Value | 3,208 |  | 3,608 |  |  |
Total available for sale, Amortized Cost | 1,470,034 |  | 1,308,019 |  |  |
Available-for-sale, Allowance for Credit Loss | (24) |  | (18) |  |  |
Total available for sale, Gross Unrealized Gains | 358 |  | 395 |  |  |
Total available for sale, Gross Unrealized Losses | (170,792) |  | (171,595) |  |  |
Total available for sale, Fair Value | 1,299,576 |  | 1,136,801 |  |  |
Total fixed maturity securities available for sale | 1,299,576 |  | 1,136,801 |  |  |
U.S. government and government agency |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,306,926 |  | 960,479 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,917 |  | 937 |  |  |
Total available for sale, Gross Unrealized Losses | (70,726) |  | (69,158) |  |  |
Total available for sale, Fair Value | 1,238,117 |  | 892,258 |  |  |
Total fixed maturity securities available for sale | 1,238,117 |  | 892,258 |  |  |
Special revenue |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,741,545 |  | 1,837,309 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 3,632 |  | 3,662 |  |  |
Total available for sale, Gross Unrealized Losses | (103,268) |  | (119,474) |  |  |
Total available for sale, Fair Value | 1,641,909 |  | 1,721,497 |  |  |
Total fixed maturity securities available for sale | 1,641,909 |  | 1,721,497 |  |  |
State general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 369,064 |  | 387,709 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,042 |  | 2,651 |  |  |
Total available for sale, Gross Unrealized Losses | (17,958) |  | (21,335) |  |  |
Total available for sale, Fair Value | 353,148 |  | 369,025 |  |  |
Total fixed maturity securities available for sale | 353,148 |  | 369,025 |  |  |
Pre-refunded |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 105,615 |  | 156,106 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,780 |  | 2,741 |  |  |
Total available for sale, Gross Unrealized Losses | (309) |  | (7) |  |  |
Total available for sale, Fair Value | 107,086 |  | 158,840 |  |  |
Total fixed maturity securities available for sale | 107,086 |  | 158,840 |  |  |
Corporate backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 197,728 |  | 210,228 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 290 |  | 334 |  |  |
Total available for sale, Gross Unrealized Losses | (9,277) |  | (10,923) |  |  |
Total available for sale, Fair Value | 188,741 |  | 199,639 |  |  |
Total fixed maturity securities available for sale | 188,741 |  | 199,639 |  |  |
Local general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 423,716 |  | 454,983 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,627 |  | 2,967 |  |  |
Total available for sale, Gross Unrealized Losses | (14,183) |  | (16,853) |  |  |
Total available for sale, Fair Value | 412,160 |  | 441,097 |  |  |
Total fixed maturity securities available for sale | 412,160 |  | 441,097 |  |  |
Commercial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 624,077 |  | 547,757 |  |  |
Available-for-sale, Allowance for Credit Loss | (861) |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 117 |  | 215 |  |  |
Total available for sale, Gross Unrealized Losses | (19,237) |  | (19,363) |  |  |
Total available for sale, Fair Value | 604,096 |  | 528,609 |  |  |
Total fixed maturity securities available for sale | 604,096 |  | 528,609 |  |  |
Mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 2,094,111 |  | 1,855,776 |  |  |
Available-for-sale, Allowance for Credit Loss | (885) | (23) | (18) |  |  |
Total available for sale, Gross Unrealized Gains | 475 |  | 610 |  |  |
Total available for sale, Gross Unrealized Losses | (190,029) |  | (190,958) |  |  |
Total available for sale, Fair Value | 1,903,672 |  | 1,665,410 |  |  |
Total fixed maturity securities available for sale | 1,903,672 |  | 1,665,410 |  |  |
Asset-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,861,009 |  | 4,132,365 |  |  |
Available-for-sale, Allowance for Credit Loss | (1,444) | 0 | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 274 |  | 2,730 |  |  |
Total available for sale, Gross Unrealized Losses | (116,036) |  | (152,322) |  |  |
Total available for sale, Fair Value | 3,743,803 |  | 3,982,773 |  |  |
Total fixed maturity securities available for sale | 3,743,803 |  | 3,982,773 |  |  |
Industrial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,502,256 |  | 3,491,645 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | (1,704) |  |  |
Total available for sale, Gross Unrealized Gains | 5,327 |  | 4,439 |  |  |
Total available for sale, Gross Unrealized Losses | (207,655) |  | (241,381) |  |  |
Total available for sale, Fair Value | 3,299,928 |  | 3,252,999 |  |  |
Total fixed maturity securities available for sale | 3,299,928 |  | 3,252,999 |  |  |
Financial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 2,749,952 |  | 2,585,247 |  |  |
Available-for-sale, Allowance for Credit Loss | (8,867) |  | (2,997) |  |  |
Total available for sale, Gross Unrealized Gains | 2,178 |  | 5,505 |  |  |
Total available for sale, Gross Unrealized Losses | (125,163) |  | (117,383) |  |  |
Total available for sale, Fair Value | 2,618,100 |  | 2,470,372 |  |  |
Total fixed maturity securities available for sale | 2,618,100 |  | 2,470,372 |  |  |
Utilities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 654,356 |  | 586,066 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,555 |  | 1,307 |  |  |
Total available for sale, Gross Unrealized Losses | (34,486) |  | (36,325) |  |  |
Total available for sale, Fair Value | 621,425 |  | 551,048 |  |  |
Total fixed maturity securities available for sale | 621,425 |  | 551,048 |  |  |
Other |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 478,682 |  | 441,230 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 13 |  | 0 |  |  |
Total available for sale, Gross Unrealized Losses | (11,080) |  | (11,657) |  |  |
Total available for sale, Fair Value | 467,615 |  | 429,573 |  |  |
Total fixed maturity securities available for sale | 467,615 |  | 429,573 |  |  |
Corporate |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 7,385,246 |  | 7,104,188 |  |  |
Available-for-sale, Allowance for Credit Loss | (8,867) | (3,795) | (4,701) | (182) | 0 | (16)
Total available for sale, Gross Unrealized Gains | 9,073 |  | 11,251 |  |  |
Total available for sale, Gross Unrealized Losses | (378,384) |  | (406,746) |  |  |
Total available for sale, Fair Value | 7,007,068 |  | 6,703,992 |  |  |
Total fixed maturity securities available for sale | 7,007,068 |  | 6,703,992 |  |  |
Foreign government |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,567,024 |  | 1,564,930 |  |  |
Available-for-sale, Allowance for Credit Loss | (33,052) | $ (33,324) | (32,633) | $ (33,096) | $ (26,153) | $ (22,222)
Total available for sale, Gross Unrealized Gains | 2,719 |  | 4,283 |  |  |
Total available for sale, Gross Unrealized Losses | (129,083) |  | (135,058) |  |  |
Total available for sale, Fair Value | 1,407,608 |  | 1,401,522 |  |  |
Total fixed maturity securities available for sale | $ 1,407,608 |  | $ 1,401,522 |  |  |

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