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Investment Information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months |  | $ 4,844,857 | $ 4,280,273
Estimated Fair Value - 12 Months or More |  | 47,077 | 158,538
Estimated Fair Value |  | 4,891,934 | 4,438,811
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months |  | (90,903) | (52,926)
Gross Unrealized Losses - 12 Months or More |  | (3,828) | (2,077)
Gross Unrealized Losses |  | (94,731) | (55,003)
Fixed maturities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 4,762,893 | 4,184,496
Estimated Fair Value - 12 Months or More | [1] | 47,077 | 158,538
Estimated Fair Value | [1] | 4,809,970 | 4,343,034
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (89,949) | (51,378)
Gross Unrealized Losses - 12 Months or More | [1] | (3,828) | (2,077)
Gross Unrealized Losses | [1] | (93,777) | (53,455)
Fixed maturities | Corporate bonds |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 1,414,931 | 675,131
Estimated Fair Value - 12 Months or More | [1] | 4,169 | 37,671
Estimated Fair Value | [1] | 1,419,100 | 712,802
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (38,353) | (12,350)
Gross Unrealized Losses - 12 Months or More | [1] | (2,095) | (443)
Gross Unrealized Losses | [1] | (40,448) | (12,793)
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] | 186,860 | 102,887
Estimated Fair Value - 12 Months or More | [1] | 690 | 203
Estimated Fair Value | [1] | 187,550 | 103,090
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (3,262) | (927)
Gross Unrealized Losses - 12 Months or More | [1] | (111) | (4)
Gross Unrealized Losses | [1] | (3,373) | (931)
Fixed maturities | Municipal bonds |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 36,469 | 220,296
Estimated Fair Value - 12 Months or More | [1] | 0 | 0
Estimated Fair Value | [1] | 36,469 | 220,296
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (877) | (2,213)
Gross Unrealized Losses - 12 Months or More | [1] | 0 | 0
Gross Unrealized Losses | [1] | (877) | (2,213)
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] | 179,780 | 147,290
Estimated Fair Value - 12 Months or More | [1] | 2,850 | 2,683
Estimated Fair Value | [1] | 182,630 | 149,973
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (4,601) | (2,302)
Gross Unrealized Losses - 12 Months or More | [1] | (59) | (28)
Gross Unrealized Losses | [1] | (4,660) | (2,330)
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] | 1,386,590 | 1,373,127
Estimated Fair Value - 12 Months or More | [1] | 0 | 32,058
Estimated Fair Value | [1] | 1,386,590 | 1,405,185
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (4,396) | (10,089)
Gross Unrealized Losses - 12 Months or More | [1] | 0 | (45)
Gross Unrealized Losses | [1] | (4,396) | (10,134)
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] | 921,890 | 1,224,243
Estimated Fair Value - 12 Months or More | [1] | 0 | 37,610
Estimated Fair Value | [1] | 921,890 | 1,261,853
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (18,294) | (20,163)
Gross Unrealized Losses - 12 Months or More | [1] | 0 | (167)
Gross Unrealized Losses | [1] | (18,294) | (20,330)
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] | 636,373 | 441,522
Estimated Fair Value - 12 Months or More | [1] | 39,368 | 48,313
Estimated Fair Value | [1] | 675,741 | 489,835
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months | [1] | (20,166) | (3,334)
Gross Unrealized Losses - 12 Months or More | [1] | (1,563) | (1,390)
Gross Unrealized Losses | [1] | (21,729) | (4,724)
Short-term investments |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] |  |  |
Estimated Fair Value - Less than 12 Months |  | 81,964 | 95,777
Estimated Fair Value - 12 Months or More |  | 0 | 0
Estimated Fair Value |  | 81,964 | 95,777
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Gross Unrealized Losses - Less than 12 Months |  | (954) | (1,548)
Gross Unrealized Losses - 12 Months or More |  | 0 | 0
Gross Unrealized Losses |  | $ (954) | $ (1,548)
[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=121582814&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=121645371&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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