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Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
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Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Total held to maturity, Amortized Cost | $ 51,410 | $ 74,368 |
Held-to-maturity, allowance for credit loss | (114) | (387) | $ (798)
Total held to maturity, Gross Unrealized Gains | 4,277 | 11,445 |
Total held to maturity, Gross Unrealized Losses | 0 | 0 |
Total held to maturity, Fair Value | 55,573 | 85,426 |
Total held to maturity, Carrying Value | 51,296 | 73,981 |
Fixed maturity securities available for sale at cost | 18,664,073 | 16,396,936 |
Cumulative effect adjustment resulting from changes in accounting principles | (37,352) | (22,238) | (1,782)
Total available for sale, Gross Unrealized Gains | 32,166 | 278,207 |
Total available for sale, Gross Unrealized Losses | (1,122,834) | (124,213) |
Total available for sale, Fair Value | 17,536,053 | 16,528,692 |
Total available for sale, Carrying Value | 17,536,053 | 16,528,692 |
Fixed maturity securities, amortized cost | 18,715,483 | 16,471,304 |
Total investments in fixed maturity securities, Gross Unrealized Gains | 36,443 | 289,652 |
Total investments in fixed maturity securities, Gross Unrealized Losses | (1,122,834) | (124,213) |
Total investments in fixed maturity securities, Fair Value | 17,591,626 | 16,614,118 |
Total investments in fixed maturity securities, carrying value | 17,587,349 | 16,602,673 |
Fixed maturity securities, allowance for credit loss | (37,466) | (22,625) |
State and municipal |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Total held to maturity, Amortized Cost | 47,802 | 69,539 |
Held-to-maturity, allowance for credit loss | (114) | (387) |
Total held to maturity, Gross Unrealized Gains | 4,239 | 10,813 |
Total held to maturity, Gross Unrealized Losses | 0 | 0 |
Total held to maturity, Fair Value | 51,927 | 79,965 |
Total held to maturity, Carrying Value | 47,688 | 69,152 |
Fixed maturity securities available for sale at cost | 3,046,335 | 3,176,042 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 12,355 | 137,650 |
Total available for sale, Gross Unrealized Losses | (168,592) | (9,559) |
Total available for sale, Fair Value | 2,890,098 | 3,304,133 |
Total available for sale, Carrying Value | 2,890,098 | 3,304,133 |
Residential mortgage-backed |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Total held to maturity, Amortized Cost | 3,608 | 4,829 |
Held-to-maturity, allowance for credit loss | 0 | 0 |
Total held to maturity, Gross Unrealized Gains | 38 | 632 |
Total held to maturity, Gross Unrealized Losses | 0 | 0 |
Total held to maturity, Fair Value | 3,646 | 5,461 |
Total held to maturity, Carrying Value | 3,608 | 4,829 |
Fixed maturity securities available for sale at cost | 1,308,019 | 940,744 |
Cumulative effect adjustment resulting from changes in accounting principles | (18) | 0 |
Total available for sale, Gross Unrealized Gains | 395 | 9,896 |
Total available for sale, Gross Unrealized Losses | (171,595) | (11,321) |
Total available for sale, Fair Value | 1,136,801 | 939,319 |
Total available for sale, Carrying Value | 1,136,801 | 939,319 |
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 | 960,479 | 851,128 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 937 | 8,509 |
Total available for sale, Gross Unrealized Losses | (69,158) | (4,294) |
Total available for sale, Fair Value | 892,258 | 855,343 |
Total available for sale, Carrying Value | 892,258 | 855,343 |
Special Revenue |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 1,837,309 | 2,016,382 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 3,662 | 62,961 |
Total available for sale, Gross Unrealized Losses | (119,474) | (5,706) |
Total available for sale, Fair Value | 1,721,497 | 2,073,637 |
Total available for sale, Carrying Value | 1,721,497 | 2,073,637 |
Debt, State Government |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 387,709 | 388,110 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 2,651 | 23,152 |
Total available for sale, Gross Unrealized Losses | (21,335) | (1,015) |
Total available for sale, Fair Value | 369,025 | 410,247 |
Total available for sale, Carrying Value | 369,025 | 410,247 |
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 | 156,106 | 202,633 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 2,741 | 14,891 |
Total available for sale, Gross Unrealized Losses | (7) | (574) |
Total available for sale, Fair Value | 158,840 | 216,950 |
Total available for sale, Carrying Value | 158,840 | 216,950 |
Corporation |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 210,228 | 166,943 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 334 | 7,191 |
Total available for sale, Gross Unrealized Losses | (10,923) | (1,532) |
Total available for sale, Fair Value | 199,639 | 172,602 |
Total available for sale, Carrying Value | 199,639 | 172,602 |
Debt, Local Government |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 454,983 | 401,974 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 2,967 | 29,455 |
Total available for sale, Gross Unrealized Losses | (16,853) | (732) |
Total available for sale, Fair Value | 441,097 | 430,697 |
Total available for sale, Carrying Value | 441,097 | 430,697 |
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 | 547,757 | 125,709 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 215 | 3,388 |
Total available for sale, Gross Unrealized Losses | (19,363) | (341) |
Total available for sale, Fair Value | 528,609 | 128,756 |
Total available for sale, Carrying Value | 528,609 | 128,756 |
Mortgage- Backed |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 1,855,776 | 1,066,453 |
Cumulative effect adjustment resulting from changes in accounting principles | (18) | 0 |
Total available for sale, Gross Unrealized Gains | 610 | 13,284 |
Total available for sale, Gross Unrealized Losses | (190,958) | (11,662) |
Total available for sale, Fair Value | 1,665,410 | 1,068,075 |
Total available for sale, Carrying Value | 1,665,410 | 1,068,075 |
Asset-backed Securities |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 4,132,365 | 4,504,950 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 2,730 | 4,409 |
Total available for sale, Gross Unrealized Losses | (152,322) | (18,794) |
Total available for sale, Fair Value | 3,982,773 | 4,490,565 |
Total available for sale, Carrying Value | 3,982,773 | 4,490,565 |
Industrial Property |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 3,491,645 | 3,231,520 |
Cumulative effect adjustment resulting from changes in accounting principles | (1,704) | (16) |
Total available for sale, Gross Unrealized Gains | 4,439 | 62,751 |
Total available for sale, Gross Unrealized Losses | (241,381) | (21,092) |
Total available for sale, Fair Value | 3,252,999 | 3,273,163 |
Total available for sale, Carrying Value | 3,252,999 | 3,273,163 |
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 | 2,585,247 | 1,739,282 |
Cumulative effect adjustment resulting from changes in accounting principles | (2,997) | 0 |
Total available for sale, Gross Unrealized Gains | 5,505 | 30,709 |
Total available for sale, Gross Unrealized Losses | (117,383) | (6,591) |
Total available for sale, Fair Value | 2,470,372 | 1,763,400 |
Total available for sale, Carrying Value | 2,470,372 | 1,763,400 |
Public Utility, Bonds |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 586,066 | 396,242 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 1,307 | 13,262 |
Total available for sale, Gross Unrealized Losses | (36,325) | (3,202) |
Total available for sale, Fair Value | 551,048 | 406,302 |
Total available for sale, Carrying Value | 551,048 | 406,302 |
Other Security Investments |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 441,230 | 154,210 |
Cumulative effect adjustment resulting from changes in accounting principles | 0 | 0 |
Total available for sale, Gross Unrealized Gains | 0 | 125 |
Total available for sale, Gross Unrealized Losses | (11,657) | (1,525) |
Total available for sale, Fair Value | 429,573 | 152,810 |
Total available for sale, Carrying Value | 429,573 | 152,810 |
Corporate |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 7,104,188 | 5,521,254 |
Cumulative effect adjustment resulting from changes in accounting principles | (4,701) | (16) | (518)
Total available for sale, Gross Unrealized Gains | 11,251 | 106,847 |
Total available for sale, Gross Unrealized Losses | (406,746) | (32,410) |
Total available for sale, Fair Value | 6,703,992 | 5,595,675 |
Total available for sale, Carrying Value | 6,703,992 | 5,595,675 |
Foreign government |  |  |
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items] |  |  |
Fixed maturity securities available for sale at cost | 1,564,930 | 1,277,109 |
Cumulative effect adjustment resulting from changes in accounting principles | (32,633) | (22,222) | $ (1,264)
Total available for sale, Gross Unrealized Gains | 4,283 | 7,508 |
Total available for sale, Gross Unrealized Losses | (135,058) | (47,494) |
Total available for sale, Fair Value | 1,401,522 | 1,214,901 |
Total available for sale, Carrying Value | $ 1,401,522 | $ 1,214,901 |

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