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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands | Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021 | Jun. 30, 2021 | Mar. 31, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | $ 50,641 |  | $ 74,368 |  |  |
Held-to-maturity, Allowance for Credit Loss | (127) | $ (378) | (387) | $ (453) | $ (730) | $ (798)
Held to maturity, Gross Unrealized Gains | 5,517 |  | 11,445 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 56,031 |  | 85,426 |  |  |
Total held to maturity, Carrying Value | 50,514 |  | 73,981 |  |  |
Total available for sale, Amortized Cost | 17,646,779 |  | 16,396,936 |  |  |
Available-for-sale, Allowance for Credit Loss | (33,278) | (26,153) | (22,238) | (18,911) | (20,009) | (1,782)
Total available for sale, Gross Unrealized Gains | 27,704 |  | 278,207 |  |  |
Total available for sale, Gross Unrealized Losses | (840,205) |  | (124,213) |  |  |
Total available for sale, Fair Value | 16,801,000 |  | 16,528,692 |  |  |
Total available for sale, Carrying Value | 16,801,000 |  | 16,528,692 |  |  |
Total investments in fixed maturity securities, Amortized Value | 17,697,420 |  | 16,471,304 |  |  |
Fixed maturity securities, allowance for credit loss | (33,405) |  | (22,625) |  |  |
Total investments in fixed maturity securities, Gross Unrealized Gains | 33,221 |  | 289,652 |  |  |
Total investments in fixed maturity securities, Gross Unrealized Losses | (840,205) |  | (124,213) |  |  |
Total investments in fixed maturity securities, Fair Value | 16,857,031 |  | 16,614,118 |  |  |
Total investments in fixed maturity securities, Carrying Value | 16,851,514 |  | 16,602,673 |  |  |
State and municipal |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 46,486 |  | 69,539 |  |  |
Held-to-maturity, Allowance for Credit Loss | (127) |  | (387) |  |  |
Held to maturity, Gross Unrealized Gains | 5,347 |  | 10,813 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 51,706 |  | 79,965 |  |  |
Total held to maturity, Carrying Value | 46,359 |  | 69,152 |  |  |
Total available for sale, Amortized Cost | 3,101,679 |  | 3,176,042 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 20,871 |  | 137,650 |  |  |
Total available for sale, Gross Unrealized Losses | (105,287) |  | (9,559) |  |  |
Total available for sale, Fair Value | 3,017,263 |  | 3,304,133 |  |  |
Total available for sale, Carrying Value | 3,017,263 |  | 3,304,133 |  |  |
Residential mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total held to maturity, Amortized Cost | 4,155 |  | 4,829 |  |  |
Held-to-maturity, Allowance for Credit Loss | 0 |  | 0 |  |  |
Held to maturity, Gross Unrealized Gains | 170 |  | 632 |  |  |
Held to maturity, Gross Unrealized Losses | 0 |  | 0 |  |  |
Total held to maturity, Fair Value | 4,325 |  | 5,461 |  |  |
Total held to maturity, Carrying Value | 4,155 |  | 4,829 |  |  |
Total available for sale, Amortized Cost | 1,181,482 |  | 940,744 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 794 |  | 9,896 |  |  |
Total available for sale, Gross Unrealized Losses | (110,774) |  | (11,321) |  |  |
Total available for sale, Fair Value | 1,071,502 |  | 939,319 |  |  |
Total available for sale, Carrying Value | 1,071,502 |  | 939,319 |  |  |
U.S. government and government agency |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 762,615 |  | 851,128 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,213 |  | 8,509 |  |  |
Total available for sale, Gross Unrealized Losses | (42,914) |  | (4,294) |  |  |
Total available for sale, Fair Value | 720,914 |  | 855,343 |  |  |
Total available for sale, Carrying Value | 720,914 |  | 855,343 |  |  |
Special revenue |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,937,329 |  | 2,016,382 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 6,636 |  | 62,961 |  |  |
Total available for sale, Gross Unrealized Losses | (76,329) |  | (5,706) |  |  |
Total available for sale, Fair Value | 1,867,636 |  | 2,073,637 |  |  |
Total available for sale, Carrying Value | 1,867,636 |  | 2,073,637 |  |  |
State general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 378,081 |  | 388,110 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 3,957 |  | 23,152 |  |  |
Total available for sale, Gross Unrealized Losses | (13,458) |  | (1,015) |  |  |
Total available for sale, Fair Value | 368,580 |  | 410,247 |  |  |
Total available for sale, Carrying Value | 368,580 |  | 410,247 |  |  |
Pre-refunded |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 200,716 |  | 202,633 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 3,232 |  | 14,891 |  |  |
Total available for sale, Gross Unrealized Losses | (49) |  | (574) |  |  |
Total available for sale, Fair Value | 203,899 |  | 216,950 |  |  |
Total available for sale, Carrying Value | 203,899 |  | 216,950 |  |  |
Corporate backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 166,591 |  | 166,943 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 453 |  | 7,191 |  |  |
Total available for sale, Gross Unrealized Losses | (6,836) |  | (1,532) |  |  |
Total available for sale, Fair Value | 160,208 |  | 172,602 |  |  |
Total available for sale, Carrying Value | 160,208 |  | 172,602 |  |  |
Local general obligation |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 418,962 |  | 401,974 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 6,593 |  | 29,455 |  |  |
Total available for sale, Gross Unrealized Losses | (8,615) |  | (732) |  |  |
Total available for sale, Fair Value | 416,940 |  | 430,697 |  |  |
Total available for sale, Carrying Value | 416,940 |  | 430,697 |  |  |
Commercial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 413,090 |  | 125,709 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 552 |  | 3,388 |  |  |
Total available for sale, Gross Unrealized Losses | (8,045) |  | (341) |  |  |
Total available for sale, Fair Value | 405,597 |  | 128,756 |  |  |
Total available for sale, Carrying Value | 405,597 |  | 128,756 |  |  |
Mortgage-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,594,572 |  | 1,066,453 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 1,346 |  | 13,284 |  |  |
Total available for sale, Gross Unrealized Losses | (118,819) |  | (11,662) |  |  |
Total available for sale, Fair Value | 1,477,099 |  | 1,068,075 |  |  |
Total available for sale, Carrying Value | 1,477,099 |  | 1,068,075 |  |  |
Asset-backed |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 4,421,553 |  | 4,504,950 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 484 |  | 4,409 |  |  |
Total available for sale, Gross Unrealized Losses | (129,549) |  | (18,794) |  |  |
Total available for sale, Fair Value | 4,292,488 |  | 4,490,565 |  |  |
Total available for sale, Carrying Value | 4,292,488 |  | 4,490,565 |  |  |
Industrial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 3,412,383 |  | 3,231,520 |  |  |
Available-for-sale, Allowance for Credit Loss | (164) |  | (16) |  |  |
Total available for sale, Gross Unrealized Gains | 2,643 |  | 62,751 |  |  |
Total available for sale, Gross Unrealized Losses | (199,225) |  | (21,092) |  |  |
Total available for sale, Fair Value | 3,215,637 |  | 3,273,163 |  |  |
Total available for sale, Carrying Value | 3,215,637 |  | 3,273,163 |  |  |
Financial |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,909,682 |  | 1,739,282 |  |  |
Available-for-sale, Allowance for Credit Loss | (18) |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 341 |  | 30,709 |  |  |
Total available for sale, Gross Unrealized Losses | (98,344) |  | (6,591) |  |  |
Total available for sale, Fair Value | 1,811,661 |  | 1,763,400 |  |  |
Total available for sale, Carrying Value | 1,811,661 |  | 1,763,400 |  |  |
Utilities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 448,342 |  | 396,242 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 249 |  | 13,262 |  |  |
Total available for sale, Gross Unrealized Losses | (26,490) |  | (3,202) |  |  |
Total available for sale, Fair Value | 422,101 |  | 406,302 |  |  |
Total available for sale, Carrying Value | 422,101 |  | 406,302 |  |  |
Other |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 247,531 |  | 154,210 |  |  |
Available-for-sale, Allowance for Credit Loss | 0 |  | 0 |  |  |
Total available for sale, Gross Unrealized Gains | 29 |  | 125 |  |  |
Total available for sale, Gross Unrealized Losses | (7,834) |  | (1,525) |  |  |
Total available for sale, Fair Value | 239,726 |  | 152,810 |  |  |
Total available for sale, Carrying Value | 239,726 |  | 152,810 |  |  |
Corporate |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 6,017,938 |  | 5,521,254 |  |  |
Available-for-sale, Allowance for Credit Loss | (182) | 0 | (16) | (12) | (16) | (518)
Total available for sale, Gross Unrealized Gains | 3,262 |  | 106,847 |  |  |
Total available for sale, Gross Unrealized Losses | (331,893) |  | (32,410) |  |  |
Total available for sale, Fair Value | 5,689,125 |  | 5,595,675 |  |  |
Total available for sale, Carrying Value | 5,689,125 |  | 5,595,675 |  |  |
Foreign government |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Total available for sale, Amortized Cost | 1,748,422 |  | 1,277,109 |  |  |
Available-for-sale, Allowance for Credit Loss | (33,096) | $ (26,153) | (22,222) | $ (18,899) | $ (19,993) | $ (1,264)
Total available for sale, Gross Unrealized Gains | 528 |  | 7,508 |  |  |
Total available for sale, Gross Unrealized Losses | (111,743) |  | (47,494) |  |  |
Total available for sale, Fair Value | 1,604,111 |  | 1,214,901 |  |  |
Total available for sale, Carrying Value | $ 1,604,111 |  | $ 1,214,901 |  |  |

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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 -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + Details
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- Definition Amount of allowance for credit loss on 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 -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922900-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922900-210455 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454 + Details
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- Definition Amount, after allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity) and 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 -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
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- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity) and 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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=SL120269820-111563 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + Details
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- Definition Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + Details
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- Definition Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=SL120269820-111563 + Details
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- Definition Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123581744&loc=SL120269820-111563 + Details
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- Definition Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity). + References No definition available. + Details
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- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123581744&loc=SL120269820-111563 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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