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Investment Information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months |  | $ 8,022,408 | $ 3,788,763
Estimated Fair Value - 12 Months or More |  | 158,155 | 70,404
Estimated Fair Value |  | 8,180,563 | 3,859,167
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months |  | (72,763) | (76,067)
Gross Unrealized Losses - 12 Months or More |  | (5,654) | (3,587)
Gross Unrealized Losses |  | (78,417) | (79,654)
Fixed maturities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 7,491,278 | 3,690,843
Estimated Fair Value - 12 Months or More | [1] | 158,155 | 70,404
Estimated Fair Value | [1] | 7,649,433 | 3,761,247
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (71,357) | (74,004)
Gross Unrealized Losses - 12 Months or More | [1] | (5,654) | (3,587)
Gross Unrealized Losses | [1] | (77,011) | (77,591)
Fixed maturities | Corporate bonds |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 2,264,801 | 747,442
Estimated Fair Value - 12 Months or More | [1] | 22,895 | 3,934
Estimated Fair Value | [1] | 2,287,696 | 751,376
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (32,733) | (33,086)
Gross Unrealized Losses - 12 Months or More | [1] | (2,018) | (1,302)
Gross Unrealized Losses | [1] | (34,751) | (34,388)
Fixed maturities | Mortgage backed securities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 196,765 | 284,619
Estimated Fair Value - 12 Months or More | [1] | 13,632 | 3,637
Estimated Fair Value | [1] | 210,397 | 288,256
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (2,893) | (4,788)
Gross Unrealized Losses - 12 Months or More | [1] | (586) | (240)
Gross Unrealized Losses | [1] | (3,479) | (5,028)
Fixed maturities | Municipal bonds |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 24,711 | 67,937
Estimated Fair Value - 12 Months or More | [1] | 2,859 | 0
Estimated Fair Value | [1] | 27,570 | 67,937
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (1,612) | (3,835)
Gross Unrealized Losses - 12 Months or More | [1] | (48) | 0
Gross Unrealized Losses | [1] | (1,660) | (3,835)
Fixed maturities | Commercial mortgage backed securities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 20,039 | 126,624
Estimated Fair Value - 12 Months or More | [1] | 7,961 | 2,655
Estimated Fair Value | [1] | 28,000 | 129,279
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (180) | (2,916)
Gross Unrealized Losses - 12 Months or More | [1] | (431) | (38)
Gross Unrealized Losses | [1] | (611) | (2,954)
Fixed maturities | US government and government agencies |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 2,920,944 | 1,285,907
Estimated Fair Value - 12 Months or More | [1] | 0 | 0
Estimated Fair Value | [1] | 2,920,944 | 1,285,907
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (14,013) | (12,611)
Gross Unrealized Losses - 12 Months or More | [1] | 0 | 0
Gross Unrealized Losses | [1] | (14,013) | (12,611)
Fixed maturities | Non-US government securities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 1,282,422 | 543,844
Estimated Fair Value - 12 Months or More | [1] | 16,885 | 2,441
Estimated Fair Value | [1] | 1,299,307 | 546,285
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (17,562) | (7,658)
Gross Unrealized Losses - 12 Months or More | [1] | (879) | (402)
Gross Unrealized Losses | [1] | (18,441) | (8,060)
Fixed maturities | Asset backed securities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 781,596 | 634,470
Estimated Fair Value - 12 Months or More | [1] | 93,923 | 57,737
Estimated Fair Value | [1] | 875,519 | 692,207
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (2,364) | (9,110)
Gross Unrealized Losses - 12 Months or More | [1] | (1,692) | (1,605)
Gross Unrealized Losses | [1] | (4,056) | (10,715)
Short-term investments |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months |  | 531,130 | 97,920
Estimated Fair Value - 12 Months or More |  | 0 | 0
Estimated Fair Value |  | 531,130 | 97,920
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months |  | (1,406) | (2,063)
Gross Unrealized Losses - 12 Months or More |  | 0 | 0
Gross Unrealized Losses |  | $ (1,406) | $ (2,063)
[1] | In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.”

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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), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456 + Details
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