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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2022 | Jun. 30, 2022 | Dec. 31, 2021 | Sep. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | $ 50,955 |  | $ 74,368 |  |  |
Held-to-maturity, Allowance for Credit Loss | (121) | $ (127) | (387) | $ (396) | $ (453) | $ (798)
Held to maturity, Gross Unrealized Gains | 3,550 |  | 11,445 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 54,384 |  | 85,426 |  |  |
Total held to maturity, Carrying Value | 50,834 |  | 73,981 |  |  |
Total available for sale, Amortized Cost | 18,416,706 |  | 16,396,936 |  |  |
Available-for-sale, Allowance for Credit Loss | (34,595) | (33,278) | (22,238) | (16,887) | (18,911) | (1,782)
Total available for sale, Gross Unrealized Gains | 15,955 |  | 278,207 |  |  |
Total available for sale, Gross Unrealized Losses | (1,217,964) |  | (124,213) |  |  |
Total available for sale, Fair Value | 17,180,102 |  | 16,528,692 |  |  |
Total available for sale, Carrying Value | 17,180,102 |  | 16,528,692 |  |  |
Total investments in fixed maturity securities, Amortized Value | 18,467,661 |  | 16,471,304 |  |  |
Fixed maturity securities, allowance for credit loss | (34,716) |  | (22,625) |  |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 19,505 |  | 289,652 |  |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (1,217,964) |  | (124,213) |  |  |
Total investments in fixed maturity securities, Fair Value | 17,234,486 |  | 16,614,118 |  |  |
Total investments in fixed maturity securities, Carrying Value | 17,230,936 |  | 16,602,673 |  |  |
State and municipal |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 47,139 |  | 69,539 |  |  |
Held-to-maturity, Allowance for Credit Loss | (121) |  | (387) |  |  |
Held to maturity, Gross Unrealized Gains | 3,522 |  | 10,813 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 50,540 |  | 79,965 |  |  |
Total held to maturity, Carrying Value | 47,018 |  | 69,152 |  |  |
Total available for sale, Amortized Cost | 3,049,021 |  | 3,176,042 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 8,723 |  | 137,650 |  |  |
Total available for sale, Gross Unrealized Losses | (184,620) |  | (9,559) |  |  |
Total available for sale, Fair Value | 2,873,124 |  | 3,304,133 |  |  |
Total available for sale, Carrying Value | 2,873,124 |  | 3,304,133 |  |  |
Residential mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 3,816 |  | 4,829 |  |  |
Held-to-maturity, Allowance for Credit Loss | 0 |  | 0 |  |  |
Held to maturity, Gross Unrealized Gains | 28 |  | 632 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 3,844 |  | 5,461 |  |  |
Total held to maturity, Carrying Value | 3,816 |  | 4,829 |  |  |
Total available for sale, Amortized Cost | 1,297,413 |  | 940,744 |  |  |
Available-for-sale, Allowance for Credit Loss | (21) |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 210 |  | 9,896 |  |  |
Total available for sale, Gross Unrealized Losses | (163,514) |  | (11,321) |  |  |
Total available for sale, Fair Value | 1,134,088 |  | 939,319 |  |  |
Total available for sale, Carrying Value | 1,134,088 |  | 939,319 |  |  |
U.S. government and government agency |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 937,214 |  | 851,128 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,185 |  | 8,509 |  |  |
Total available for sale, Gross Unrealized Losses | (68,807) |  | (4,294) |  |  |
Total available for sale, Fair Value | 870,592 |  | 855,343 |  |  |
Total available for sale, Carrying Value | 870,592 |  | 855,343 |  |  |
Special revenue |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,877,116 |  | 2,016,382 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 3,235 |  | 62,961 |  |  |
Total available for sale, Gross Unrealized Losses | (128,427) |  | (5,706) |  |  |
Total available for sale, Fair Value | 1,751,924 |  | 2,073,637 |  |  |
Total available for sale, Carrying Value | 1,751,924 |  | 2,073,637 |  |  |
State general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 366,596 |  | 388,110 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,907 |  | 23,152 |  |  |
Total available for sale, Gross Unrealized Losses | (22,956) |  | (1,015) |  |  |
Total available for sale, Fair Value | 345,547 |  | 410,247 |  |  |
Total available for sale, Carrying Value | 345,547 |  | 410,247 |  |  |
Pre-refunded |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 156,685 |  | 202,633 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 946 |  | 14,891 |  |  |
Total available for sale, Gross Unrealized Losses | (200) |  | (574) |  |  |
Total available for sale, Fair Value | 157,431 |  | 216,950 |  |  |
Total available for sale, Carrying Value | 157,431 |  | 216,950 |  |  |
Corporate backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 198,649 |  | 166,943 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 0 |  | 7,191 |  |  |
Total available for sale, Gross Unrealized Losses | (13,558) |  | (1,532) |  |  |
Total available for sale, Fair Value | 185,091 |  | 172,602 |  |  |
Total available for sale, Carrying Value | 185,091 |  | 172,602 |  |  |
Local general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 449,975 |  | 401,974 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 2,635 |  | 29,455 |  |  |
Total available for sale, Gross Unrealized Losses | (19,479) |  | (732) |  |  |
Total available for sale, Fair Value | 433,131 |  | 430,697 |  |  |
Total available for sale, Carrying Value | 433,131 |  | 430,697 |  |  |
Commercial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 474,049 |  | 125,709 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 52 |  | 3,388 |  |  |
Total available for sale, Gross Unrealized Losses | (17,164) |  | (341) |  |  |
Total available for sale, Fair Value | 456,937 |  | 128,756 |  |  |
Total available for sale, Carrying Value | 456,937 |  | 128,756 |  |  |
Mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,771,462 |  | 1,066,453 |  |  |
Available-for-sale, Allowance for Credit Loss | (21) | 0 | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 262 |  | 13,284 |  |  |
Total available for sale, Gross Unrealized Losses | (180,678) |  | (11,662) |  |  |
Total available for sale, Fair Value | 1,591,025 |  | 1,068,075 |  |  |
Total available for sale, Carrying Value | 1,591,025 |  | 1,068,075 |  |  |
Asset-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 4,402,479 |  | 4,504,950 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 433 |  | 4,409 |  |  |
Total available for sale, Gross Unrealized Losses | (161,474) |  | (18,794) |  |  |
Total available for sale, Fair Value | 4,241,438 |  | 4,490,565 |  |  |
Total available for sale, Carrying Value | 4,241,438 |  | 4,490,565 |  |  |
Industrial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,408,585 |  | 3,231,520 |  |  |
Available-for-sale, Allowance for Credit Loss | (319) |  | (16) |  |  |
Total available for sale, Gross Unrealized Gains | 1,604 |  | 62,751 |  |  |
Total available for sale, Gross Unrealized Losses | (278,084) |  | (21,092) |  |  |
Total available for sale, Fair Value | 3,131,786 |  | 3,273,163 |  |  |
Total available for sale, Carrying Value | 3,131,786 |  | 3,273,163 |  |  |
Financial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 2,411,487 |  | 1,739,282 |  |  |
Available-for-sale, Allowance for Credit Loss | (660) |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,345 |  | 30,709 |  |  |
Total available for sale, Gross Unrealized Losses | (142,620) |  | (6,591) |  |  |
Total available for sale, Fair Value | 2,269,552 |  | 1,763,400 |  |  |
Total available for sale, Carrying Value | 2,269,552 |  | 1,763,400 |  |  |
Utilities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 499,140 |  | 396,242 |  |  |
Available-for-sale, Allowance for Credit Loss | (220) |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 0 |  | 13,262 |  |  |
Total available for sale, Gross Unrealized Losses | (40,823) |  | (3,202) |  |  |
Total available for sale, Fair Value | 458,097 |  | 406,302 |  |  |
Total available for sale, Carrying Value | 458,097 |  | 406,302 |  |  |
Other |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 357,472 |  | 154,210 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 8 |  | 125 |  |  |
Total available for sale, Gross Unrealized Losses | (12,171) |  | (1,525) |  |  |
Total available for sale, Fair Value | 345,309 |  | 152,810 |  |  |
Total available for sale, Carrying Value | 345,309 |  | 152,810 |  |  |
Corporate |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 6,676,684 |  | 5,521,254 |  |  |
Available-for-sale, Allowance for Credit Loss | (1,199) | (182) | (16) | (12) | (12) | (518)
Total available for sale, Gross Unrealized Gains | 2,957 |  | 106,847 |  |  |
Total available for sale, Gross Unrealized Losses | (473,698) |  | (32,410) |  |  |
Total available for sale, Fair Value | 6,204,744 |  | 5,595,675 |  |  |
Total available for sale, Carrying Value | 6,204,744 |  | 5,595,675 |  |  |
Foreign government |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,579,846 |  | 1,277,109 |  |  |
Available-for-sale, Allowance for Credit Loss | (33,375) | $ (33,096) | (22,222) | $ (16,875) | $ (18,899) | $ (1,264)
Total available for sale, Gross Unrealized Gains | 1,395 |  | 7,508 |  |  |
Total available for sale, Gross Unrealized Losses | (148,687) |  | (47,494) |  |  |
Total available for sale, Fair Value | 1,399,179 |  | 1,214,901 |  |  |
Total available for sale, Carrying Value | $ 1,399,179 |  | $ 1,214,901 |  |  |

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