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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 | Dec. 31, 2022
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Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total held to maturity, Amortized Cost | $ 54,020 | $ 53,415 |  |
Held-to-maturity, Allowance for Credit Loss | (38) | (43) | $ (107) | $ (114)
Held to maturity, Gross Unrealized Gains | 2,358 | 3,239 |  |
Held to maturity, Gross Unrealized Losses | 0 | 0 |  |
Total held to maturity, Fair Value | 56,340 | 56,611 |  |
Total held to maturity, Carrying Value | 53,982 | 53,372 |  |
Total available for sale, Amortized Cost | 21,721,496 | 20,861,830 |  |
Available-for-sale, Allowance for Credit Loss | (22,831) | (36,708) | (37,142) | (37,352)
Total available for sale, Gross Unrealized Gains | 73,248 | 110,989 |  |
Total available for sale, Gross Unrealized Losses | (861,041) | (811,175) |  |
Total available for sale, Fair Value | 20,910,872 | 20,124,936 |  |
Total fixed maturity securities available for sale | 20,910,872 | 20,124,936 |  |
Total investments in fixed maturity securities, Amortized Value | 21,775,516 | 20,915,245 |  |
Fixed maturity securities, allowance for credit loss | (22,869) | (36,751) |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 75,606 | 114,228 |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (861,041) | (811,175) |  |
Total investments in fixed maturity securities, Fair Value | 20,967,212 | 20,181,547 |  |
Total investments in fixed maturity securities, Carrying Value | 20,964,854 | 20,178,308 |  |
State and municipal |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total held to maturity, Amortized Cost | 51,258 | 50,547 |  |
Held-to-maturity, Allowance for Credit Loss | (38) | (43) |  |
Held to maturity, Gross Unrealized Gains | 2,276 | 3,132 |  |
Held to maturity, Gross Unrealized Losses | 0 | 0 |  |
Total held to maturity, Fair Value | 53,496 | 53,636 |  |
Total held to maturity, Carrying Value | 51,220 | 50,504 |  |
Total available for sale, Amortized Cost | 2,668,420 | 2,742,922 |  |
Available-for-sale, Allowance for Credit Loss | (693) | (757) |  |
Total available for sale, Gross Unrealized Gains | 8,948 | 14,719 |  |
Total available for sale, Gross Unrealized Losses | (123,889) | (122,462) |  |
Total available for sale, Fair Value | 2,552,786 | 2,634,422 |  |
Total fixed maturity securities available for sale | 2,552,786 | 2,634,422 |  |
Residential mortgage-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total held to maturity, Amortized Cost | 2,762 | 2,868 |  |
Held-to-maturity, Allowance for Credit Loss | 0 | 0 |  |
Held to maturity, Gross Unrealized Gains | 82 | 107 |  |
Held to maturity, Gross Unrealized Losses | 0 | 0 |  |
Total held to maturity, Fair Value | 2,844 | 2,975 |  |
Total held to maturity, Carrying Value | 2,762 | 2,868 |  |
Total available for sale, Amortized Cost | 1,963,060 | 1,773,206 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 9,805 | 12,780 |  |
Total available for sale, Gross Unrealized Losses | (174,727) | (163,844) |  |
Total available for sale, Fair Value | 1,798,138 | 1,622,142 |  |
Total fixed maturity securities available for sale | 1,798,138 | 1,622,142 |  |
U.S. government and government agency |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 1,875,920 | 1,762,997 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 8,888 | 11,403 |  |
Total available for sale, Gross Unrealized Losses | (55,256) | (57,669) |  |
Total available for sale, Fair Value | 1,829,552 | 1,716,731 |  |
Total fixed maturity securities available for sale | 1,829,552 | 1,716,731 |  |
Special revenue |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 1,626,511 | 1,682,550 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 3,370 | 5,651 |  |
Total available for sale, Gross Unrealized Losses | (83,769) | (82,006) |  |
Total available for sale, Fair Value | 1,546,112 | 1,606,195 |  |
Total fixed maturity securities available for sale | 1,546,112 | 1,606,195 |  |
State general obligation |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 393,807 | 394,429 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 1,833 | 3,550 |  |
Total available for sale, Gross Unrealized Losses | (16,316) | (16,405) |  |
Total available for sale, Fair Value | 379,324 | 381,574 |  |
Total fixed maturity securities available for sale | 379,324 | 381,574 |  |
Pre-refunded |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 88,359 | 103,029 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 908 | 1,634 |  |
Total available for sale, Gross Unrealized Losses | (214) | (185) |  |
Total available for sale, Fair Value | 89,053 | 104,478 |  |
Total fixed maturity securities available for sale | 89,053 | 104,478 |  |
Corporate backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 166,261 | 166,873 |  |
Available-for-sale, Allowance for Credit Loss | (693) | (757) |  |
Total available for sale, Gross Unrealized Gains | 539 | 696 |  |
Total available for sale, Gross Unrealized Losses | (10,588) | (11,973) |  |
Total available for sale, Fair Value | 155,519 | 154,839 |  |
Total fixed maturity securities available for sale | 155,519 | 154,839 |  |
Local general obligation |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 393,482 | 396,041 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 2,298 | 3,188 |  |
Total available for sale, Gross Unrealized Losses | (13,002) | (11,893) |  |
Total available for sale, Fair Value | 382,778 | 387,336 |  |
Total fixed maturity securities available for sale | 382,778 | 387,336 |  |
Commercial |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 630,348 | 657,157 |  |
Available-for-sale, Allowance for Credit Loss | (562) | (158) |  |
Total available for sale, Gross Unrealized Gains | 326 | 626 |  |
Total available for sale, Gross Unrealized Losses | (20,232) | (13,312) |  |
Total available for sale, Fair Value | 609,880 | 644,313 |  |
Total fixed maturity securities available for sale | 609,880 | 644,313 |  |
Mortgage-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 2,593,408 | 2,430,363 |  |
Available-for-sale, Allowance for Credit Loss | (562) | (158) | (23) | (18)
Total available for sale, Gross Unrealized Gains | 10,131 | 13,406 |  |
Total available for sale, Gross Unrealized Losses | (194,959) | (177,156) |  |
Total available for sale, Fair Value | 2,408,018 | 2,266,455 |  |
Total fixed maturity securities available for sale | 2,408,018 | 2,266,455 |  |
Asset-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 4,298,768 | 4,252,883 |  |
Available-for-sale, Allowance for Credit Loss | (1,097) | (1,164) |  |
Total available for sale, Gross Unrealized Gains | 6,475 | 8,527 |  |
Total available for sale, Gross Unrealized Losses | (64,585) | (73,206) |  |
Total available for sale, Fair Value | 4,239,561 | 4,187,040 |  |
Total fixed maturity securities available for sale | 4,239,561 | 4,187,040 |  |
Industrial |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 3,865,664 | 3,679,219 |  |
Available-for-sale, Allowance for Credit Loss | 0 | (40) |  |
Total available for sale, Gross Unrealized Gains | 14,741 | 24,312 |  |
Total available for sale, Gross Unrealized Losses | (146,130) | (143,936) |  |
Total available for sale, Fair Value | 3,734,275 | 3,559,555 |  |
Total fixed maturity securities available for sale | 3,734,275 | 3,559,555 |  |
Financial |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 3,172,945 | 2,838,220 |  |
Available-for-sale, Allowance for Credit Loss | 0 | (4,986) |  |
Total available for sale, Gross Unrealized Gains | 10,100 | 14,681 |  |
Total available for sale, Gross Unrealized Losses | (69,749) | (68,681) |  |
Total available for sale, Fair Value | 3,113,296 | 2,779,234 |  |
Total fixed maturity securities available for sale | 3,113,296 | 2,779,234 |  |
Utilities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 751,234 | 701,865 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 2,864 | 6,471 |  |
Total available for sale, Gross Unrealized Losses | (24,436) | (23,412) |  |
Total available for sale, Fair Value | 729,662 | 684,924 |  |
Total fixed maturity securities available for sale | 729,662 | 684,924 |  |
Other |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 590,644 | 635,975 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 2,516 | 1,605 |  |
Total available for sale, Gross Unrealized Losses | (7,690) | (7,234) |  |
Total available for sale, Fair Value | 585,470 | 630,346 |  |
Total fixed maturity securities available for sale | 585,470 | 630,346 |  |
Corporate |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 8,380,487 | 7,855,279 |  |
Available-for-sale, Allowance for Credit Loss | 0 | (5,026) | (3,795) | (4,701)
Total available for sale, Gross Unrealized Gains | 30,221 | 47,069 |  |
Total available for sale, Gross Unrealized Losses | (248,005) | (243,263) |  |
Total available for sale, Fair Value | 8,162,703 | 7,654,059 |  |
Total fixed maturity securities available for sale | 8,162,703 | 7,654,059 |  |
Foreign government |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 1,904,493 | 1,817,386 |  |
Available-for-sale, Allowance for Credit Loss | (20,479) | (29,603) | $ (33,324) | $ (32,633)
Total available for sale, Gross Unrealized Gains | 8,585 | 15,865 |  |
Total available for sale, Gross Unrealized Losses | (174,347) | (137,419) |  |
Total available for sale, Fair Value | 1,718,252 | 1,666,229 |  |
Total fixed maturity securities available for sale | $ 1,718,252 | $ 1,666,229 |  |

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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479130/326-30-45-1 + Details
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481830/320-10-45-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479130/326-30-45-1 + Details
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