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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Mar. 31, 2022 | Dec. 31, 2021 | Mar. 31, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total held to maturity, Amortized Cost | $ 74,620 | $ 74,368 |  |
Held-to-maturity, Allowance for Credit Loss | (378) | (387) | $ (730) | $ (798)
Held to maturity, Gross Unrealized Gains | 7,418 | 11,445 |  |
Held to maturity, Gross Unrealized Losses | 0 | 0 |  |
Total held to maturity, Fair Value | 81,660 | 85,426 |  |
Total held to maturity, Carrying Value | 74,242 | 73,981 |  |
Total available for sale, Amortized Cost | 16,764,083 | 16,396,936 |  |
Available-for-sale, Allowance for Credit Loss | (26,153) | (22,238) | (20,009) | (1,782)
Total available for sale, Gross Unrealized Gains | 67,990 | 278,207 |  |
Total available for sale, Gross Unrealized Losses | (453,966) | (124,213) |  |
Total available for sale, Fair Value | 16,351,954 | 16,528,692 |  |
Total available for sale, Carrying Value | 16,351,954 | 16,528,692 |  |
Total investments in fixed maturity securities, Amortized Value | 16,838,703 | 16,471,304 |  |
Fixed maturity securities, allowance for credit loss | (26,531) | (22,625) |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 75,408 | 289,652 |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (453,966) | (124,213) |  |
Total investments in fixed maturity securities, Fair Value | 16,433,614 | 16,614,118 |  |
Total investments in fixed maturity securities, Carrying Value | 16,426,196 | 16,602,673 |  |
State and municipal |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total held to maturity, Amortized Cost | 70,165 | 69,539 |  |
Held-to-maturity, Allowance for Credit Loss | (378) | (387) |  |
Held to maturity, Gross Unrealized Gains | 7,094 | 10,813 |  |
Held to maturity, Gross Unrealized Losses | 0 | 0 |  |
Total held to maturity, Fair Value | 76,881 | 79,965 |  |
Total held to maturity, Carrying Value | 69,787 | 69,152 |  |
Total available for sale, Amortized Cost | 3,078,004 | 3,176,042 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 38,794 | 137,650 |  |
Total available for sale, Gross Unrealized Losses | (60,924) | (9,559) |  |
Total available for sale, Fair Value | 3,055,874 | 3,304,133 |  |
Total available for sale, Carrying Value | 3,055,874 | 3,304,133 |  |
Residential mortgage-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total held to maturity, Amortized Cost | 4,455 | 4,829 |  |
Held-to-maturity, Allowance for Credit Loss | 0 | 0 |  |
Held to maturity, Gross Unrealized Gains | 324 | 632 |  |
Held to maturity, Gross Unrealized Losses | 0 | 0 |  |
Total held to maturity, Fair Value | 4,779 | 5,461 |  |
Total held to maturity, Carrying Value | 4,455 | 4,829 |  |
Total available for sale, Amortized Cost | 1,179,468 | 940,744 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 2,417 | 9,896 |  |
Total available for sale, Gross Unrealized Losses | (60,650) | (11,321) |  |
Total available for sale, Fair Value | 1,121,235 | 939,319 |  |
Total available for sale, Carrying Value | 1,121,235 | 939,319 |  |
U.S. government and government agency |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 837,167 | 851,128 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 2,397 | 8,509 |  |
Total available for sale, Gross Unrealized Losses | (20,716) | (4,294) |  |
Total available for sale, Fair Value | 818,848 | 855,343 |  |
Total available for sale, Carrying Value | 818,848 | 855,343 |  |
Special revenue |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 1,948,586 | 2,016,382 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 12,944 | 62,961 |  |
Total available for sale, Gross Unrealized Losses | (44,803) | (5,706) |  |
Total available for sale, Fair Value | 1,916,727 | 2,073,637 |  |
Total available for sale, Carrying Value | 1,916,727 | 2,073,637 |  |
State general obligation |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 378,408 | 388,110 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 7,043 | 23,152 |  |
Total available for sale, Gross Unrealized Losses | (6,574) | (1,015) |  |
Total available for sale, Fair Value | 378,877 | 410,247 |  |
Total available for sale, Carrying Value | 378,877 | 410,247 |  |
Pre-refunded |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 165,882 | 202,633 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 6,216 | 14,891 |  |
Total available for sale, Gross Unrealized Losses | 0 | (574) |  |
Total available for sale, Fair Value | 172,098 | 216,950 |  |
Total available for sale, Carrying Value | 172,098 | 216,950 |  |
Corporate backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 175,070 | 166,943 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 1,407 | 7,191 |  |
Total available for sale, Gross Unrealized Losses | (4,472) | (1,532) |  |
Total available for sale, Fair Value | 172,005 | 172,602 |  |
Total available for sale, Carrying Value | 172,005 | 172,602 |  |
Local general obligation |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 410,058 | 401,974 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 11,184 | 29,455 |  |
Total available for sale, Gross Unrealized Losses | (5,075) | (732) |  |
Total available for sale, Fair Value | 416,167 | 430,697 |  |
Total available for sale, Carrying Value | 416,167 | 430,697 |  |
Commercial |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 266,921 | 125,709 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 339 | 3,388 |  |
Total available for sale, Gross Unrealized Losses | (3,759) | (341) |  |
Total available for sale, Fair Value | 263,501 | 128,756 |  |
Total available for sale, Carrying Value | 263,501 | 128,756 |  |
Mortgage-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 1,446,389 | 1,066,453 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 2,756 | 13,284 |  |
Total available for sale, Gross Unrealized Losses | (64,409) | (11,662) |  |
Total available for sale, Fair Value | 1,384,736 | 1,068,075 |  |
Total available for sale, Carrying Value | 1,384,736 | 1,068,075 |  |
Asset-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 4,319,535 | 4,504,950 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 1,459 | 4,409 |  |
Total available for sale, Gross Unrealized Losses | (63,573) | (18,794) |  |
Total available for sale, Fair Value | 4,257,421 | 4,490,565 |  |
Total available for sale, Carrying Value | 4,257,421 | 4,490,565 |  |
Industrial |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 3,373,710 | 3,231,520 |  |
Available-for-sale, Allowance for Credit Loss |  | (16) |  |
Total available for sale, Gross Unrealized Gains | 12,708 | 62,751 |  |
Total available for sale, Gross Unrealized Losses | (115,614) | (21,092) |  |
Total available for sale, Fair Value | 3,270,804 | 3,273,163 |  |
Total available for sale, Carrying Value | 3,270,804 | 3,273,163 |  |
Financial |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 1,747,275 | 1,739,282 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 4,612 | 30,709 |  |
Total available for sale, Gross Unrealized Losses | (45,155) | (6,591) |  |
Total available for sale, Fair Value | 1,706,732 | 1,763,400 |  |
Total available for sale, Carrying Value | 1,706,732 | 1,763,400 |  |
Utilities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 422,354 | 396,242 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 1,806 | 13,262 |  |
Total available for sale, Gross Unrealized Losses | (14,434) | (3,202) |  |
Total available for sale, Fair Value | 409,726 | 406,302 |  |
Total available for sale, Carrying Value | 409,726 | 406,302 |  |
Other |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 206,526 | 154,210 |  |
Available-for-sale, Allowance for Credit Loss | 0 | 0 |  |
Total available for sale, Gross Unrealized Gains | 47 | 125 |  |
Total available for sale, Gross Unrealized Losses | (5,328) | (1,525) |  |
Total available for sale, Fair Value | 201,245 | 152,810 |  |
Total available for sale, Carrying Value | 201,245 | 152,810 |  |
Corporate |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 5,749,865 | 5,521,254 |  |
Available-for-sale, Allowance for Credit Loss | 0 | (16) | (16) | (518)
Total available for sale, Gross Unrealized Gains | 19,173 | 106,847 |  |
Total available for sale, Gross Unrealized Losses | (180,531) | (32,410) |  |
Total available for sale, Fair Value | 5,588,507 | 5,595,675 |  |
Total available for sale, Carrying Value | 5,588,507 | 5,595,675 |  |
Foreign government |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Total available for sale, Amortized Cost | 1,333,123 | 1,277,109 |  |
Available-for-sale, Allowance for Credit Loss | (26,153) | (22,222) | $ (19,993) | $ (1,264)
Total available for sale, Gross Unrealized Gains | 3,411 | 7,508 |  |
Total available for sale, Gross Unrealized Losses | (63,813) | (47,494) |  |
Total available for sale, Fair Value | 1,246,568 | 1,214,901 |  |
Total available for sale, Carrying Value | $ 1,246,568 | $ 1,214,901 |  |

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