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Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
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Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Total held to maturity, Amortized Cost | $ 53,415 | $ 51,410 |
Held-to-maturity, allowance for credit loss | (43) | (114) | $ (387)
Total held to maturity, Gross Unrealized Gains | 3,239 | 4,277 |
Total held to maturity, Gross Unrealized Losses | 0 | 0 |
Total held to maturity, Fair Value | 56,611 | 55,573 |
Total held to maturity, Carrying Value | 53,372 | 51,296 |
Fixed maturity securities available for sale at cost | 20,861,830 | 18,664,073 |
Cumulative effect adjustment resulting from changes in accounting principles | (36,708) | (37,352) | (22,238)
Total available for sale, Gross Unrealized Gains | 110,989 | 32,166 |
Total available for sale, Gross Unrealized Losses | (811,175) | (1,122,834) |
Total available for sale, Fair Value | 20,124,936 | 17,536,053 |
Total available for sale, Carrying Value | 20,124,936 | 17,536,053 |
Fixed maturity securities, amortized cost | 20,915,245 | 18,715,483 |
Total investments in fixed maturity securities, Gross Unrealized Gains | 114,228 | 36,443 |
Total investments in fixed maturity securities, Gross Unrealized Losses | (811,175) | (1,122,834) |
Total investments in fixed maturity securities, Fair Value | 20,181,547 | 17,591,626 |
Total investments in fixed maturity securities, carrying value | 20,178,308 | 17,587,349 |
Fixed maturity securities, allowance for credit loss | (36,751) | (37,466) |
State and municipal |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Total held to maturity, Amortized Cost | 50,547 | 47,802 |
Held-to-maturity, allowance for credit loss | (43) | (114) |
Total held to maturity, Gross Unrealized Gains | 3,132 | 4,239 |
Total held to maturity, Gross Unrealized Losses | 0 | 0 |
Total held to maturity, Fair Value | 53,636 | 51,927 |
Total held to maturity, Carrying Value | 50,504 | 47,688 |
Fixed maturity securities available for sale at cost | 2,742,922 | 3,046,335 |
Cumulative effect adjustment resulting from changes in accounting principles | (757) | 0 |
Total available for sale, Gross Unrealized Gains | 14,719 | 12,355 |
Total available for sale, Gross Unrealized Losses | (122,462) | (168,592) |
Total available for sale, Fair Value | 2,634,422 | 2,890,098 |
Total available for sale, Carrying Value | 2,634,422 | 2,890,098 |
Residential mortgage-backed |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Total held to maturity, Amortized Cost | 2,868 | 3,608 |
Held-to-maturity, allowance for credit loss | 0 | 0 |
Total held to maturity, Gross Unrealized Gains | 107 | 38 |
Total held to maturity, Gross Unrealized Losses | 0 | 0 |
Total held to maturity, Fair Value | 2,975 | 3,646 |
Total held to maturity, Carrying Value | 2,868 | 3,608 |
Fixed maturity securities available for sale at cost | 1,773,206 | 1,308,019 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | (18) |
Total available for sale, Gross Unrealized Gains | 12,780 | 395 |
Total available for sale, Gross Unrealized Losses | (163,844) | (171,595) |
Total available for sale, Fair Value | 1,622,142 | 1,136,801 |
Total available for sale, Carrying Value | 1,622,142 | 1,136,801 |
U.S. government and government agency |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 1,762,997 | 960,479 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 11,403 | 937 |
Total available for sale, Gross Unrealized Losses | (57,669) | (69,158) |
Total available for sale, Fair Value | 1,716,731 | 892,258 |
Total available for sale, Carrying Value | 1,716,731 | 892,258 |
Special Revenue |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 1,682,550 | 1,837,309 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 5,651 | 3,662 |
Total available for sale, Gross Unrealized Losses | (82,006) | (119,474) |
Total available for sale, Fair Value | 1,606,195 | 1,721,497 |
Total available for sale, Carrying Value | 1,606,195 | 1,721,497 |
Debt, State Government |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 394,429 | 387,709 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 3,550 | 2,651 |
Total available for sale, Gross Unrealized Losses | (16,405) | (21,335) |
Total available for sale, Fair Value | 381,574 | 369,025 |
Total available for sale, Carrying Value | 381,574 | 369,025 |
Revenue Subject to Refund |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 103,029 | 156,106 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 1,634 | 2,741 |
Total available for sale, Gross Unrealized Losses | (185) | (7) |
Total available for sale, Fair Value | 104,478 | 158,840 |
Total available for sale, Carrying Value | 104,478 | 158,840 |
Corporation |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 166,873 | 210,228 |
Cumulative effect adjustment resulting from changes in accounting principles | (757) | 0 |
Total available for sale, Gross Unrealized Gains | 696 | 334 |
Total available for sale, Gross Unrealized Losses | (11,973) | (10,923) |
Total available for sale, Fair Value | 154,839 | 199,639 |
Total available for sale, Carrying Value | 154,839 | 199,639 |
Debt, Local Government |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 396,041 | 454,983 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 3,188 | 2,967 |
Total available for sale, Gross Unrealized Losses | (11,893) | (16,853) |
Total available for sale, Fair Value | 387,336 | 441,097 |
Total available for sale, Carrying Value | 387,336 | 441,097 |
Commercial Mortgage Backed Securities |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 657,157 | 547,757 |
Cumulative effect adjustment resulting from changes in accounting principles | (158) | 0 |
Total available for sale, Gross Unrealized Gains | 626 | 215 |
Total available for sale, Gross Unrealized Losses | (13,312) | (19,363) |
Total available for sale, Fair Value | 644,313 | 528,609 |
Total available for sale, Carrying Value | 644,313 | 528,609 |
Mortgage- Backed |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 2,430,363 | 1,855,776 |
Cumulative effect adjustment resulting from changes in accounting principles | (158) | (18) | 0
Total available for sale, Gross Unrealized Gains | 13,406 | 610 |
Total available for sale, Gross Unrealized Losses | (177,156) | (190,958) |
Total available for sale, Fair Value | 2,266,455 | 1,665,410 |
Total available for sale, Carrying Value | 2,266,455 | 1,665,410 |
Asset-backed Securities |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 4,252,883 | 4,132,365 |
Cumulative effect adjustment resulting from changes in accounting principles | (1,164) | 0 |
Total available for sale, Gross Unrealized Gains | 8,527 | 2,730 |
Total available for sale, Gross Unrealized Losses | (73,206) | (152,322) |
Total available for sale, Fair Value | 4,187,040 | 3,982,773 |
Total available for sale, Carrying Value | 4,187,040 | 3,982,773 |
Industrial Property |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 3,679,219 | 3,491,645 |
Cumulative effect adjustment resulting from changes in accounting principles | (40) | (1,704) |
Total available for sale, Gross Unrealized Gains | 24,312 | 4,439 |
Total available for sale, Gross Unrealized Losses | (143,936) | (241,381) |
Total available for sale, Fair Value | 3,559,555 | 3,252,999 |
Total available for sale, Carrying Value | 3,559,555 | 3,252,999 |
Debt Security, Corporate, US |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 2,838,220 | 2,585,247 |
Cumulative effect adjustment resulting from changes in accounting principles | (4,986) | (2,997) |
Total available for sale, Gross Unrealized Gains | 14,681 | 5,505 |
Total available for sale, Gross Unrealized Losses | (68,681) | (117,383) |
Total available for sale, Fair Value | 2,779,234 | 2,470,372 |
Total available for sale, Carrying Value | 2,779,234 | 2,470,372 |
Public Utility, Bonds |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 701,865 | 586,066 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 6,471 | 1,307 |
Total available for sale, Gross Unrealized Losses | (23,412) | (36,325) |
Total available for sale, Fair Value | 684,924 | 551,048 |
Total available for sale, Carrying Value | 684,924 | 551,048 |
Other Security Investments |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 635,975 | 441,230 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 1,605 | 0 |
Total available for sale, Gross Unrealized Losses | (7,234) | (11,657) |
Total available for sale, Fair Value | 630,346 | 429,573 |
Total available for sale, Carrying Value | 630,346 | 429,573 |
Corporate |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 7,855,279 | 7,104,188 |
Cumulative effect adjustment resulting from changes in accounting principles | (5,026) | (4,701) | (16)
Total available for sale, Gross Unrealized Gains | 47,069 | 11,251 |
Total available for sale, Gross Unrealized Losses | (243,263) | (406,746) |
Total available for sale, Fair Value | 7,654,059 | 6,703,992 |
Total available for sale, Carrying Value | 7,654,059 | 6,703,992 |
Foreign government |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 1,817,386 | 1,564,930 |
Cumulative effect adjustment resulting from changes in accounting principles | (29,603) | (32,633) | $ (22,222)
Total available for sale, Gross Unrealized Gains | 15,865 | 4,283 |
Total available for sale, Gross Unrealized Losses | (137,419) | (135,058) |
Total available for sale, Fair Value | 1,666,229 | 1,401,522 |
Total available for sale, Carrying Value | $ 1,666,229 | $ 1,401,522 |

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