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Investment Information - Aging of Available For Sale Securities in an Unrealized Loss Position (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months |  | $ 3,788,763 |  | $ 4,280,273
Gross Unrealized Losses - Less than 12 Months |  | (76,067) |  | (52,926)
Estimated Fair Value - 12 Months or More |  | 70,404 |  | 158,538
Gross Unrealized Losses - 12 Months or More |  | (3,587) |  | (2,077)
Estimated Fair Value - Total |  | 3,859,167 |  | 4,438,811
Gross Unrealized Losses - Total |  | (79,654) |  | (55,003)
Fixed maturities and fixed maturities pledged under securities lending agreements |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months |  | 3,690,843 | [1] | 4,184,496
Gross Unrealized Losses - Less than 12 Months |  | (74,004) | [1] | (51,378)
Estimated Fair Value - 12 Months or More |  | 70,404 | [1] | 158,538
Gross Unrealized Losses - 12 Months or More |  | (3,587) | [1] | (2,077)
Estimated Fair Value - Total |  | 3,761,247 | [1] | 4,343,034
Gross Unrealized Losses - Total |  | (77,591) | [1] | (53,455)
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 747,442 |  | 675,131
Gross Unrealized Losses - Less than 12 Months | [1] | (33,086) |  | (12,350)
Estimated Fair Value - 12 Months or More | [1] | 3,934 |  | 37,671
Gross Unrealized Losses - 12 Months or More | [1] | (1,302) |  | (443)
Estimated Fair Value - Total | [1] | 751,376 |  | 712,802
Gross Unrealized Losses - Total | [1] | (34,388) |  | (12,793)
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 284,619 |  | 102,887
Gross Unrealized Losses - Less than 12 Months | [1] | (4,788) |  | (927)
Estimated Fair Value - 12 Months or More | [1] | 3,637 |  | 203
Gross Unrealized Losses - 12 Months or More | [1] | (240) |  | (4)
Estimated Fair Value - Total | [1] | 288,256 |  | 103,090
Gross Unrealized Losses - Total | [1] | (5,028) |  | (931)
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 67,937 |  | 220,296
Gross Unrealized Losses - Less than 12 Months | [1] | (3,835) |  | (2,213)
Estimated Fair Value - 12 Months or More | [1] | 0 |  | 0
Gross Unrealized Losses - 12 Months or More | [1] | 0 |  | 0
Estimated Fair Value - Total | [1] | 67,937 |  | 220,296
Gross Unrealized Losses - Total | [1] | (3,835) |  | (2,213)
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 126,624 |  | 147,290
Gross Unrealized Losses - Less than 12 Months | [1] | (2,916) |  | (2,302)
Estimated Fair Value - 12 Months or More | [1] | 2,655 |  | 2,683
Gross Unrealized Losses - 12 Months or More | [1] | (38) |  | (28)
Estimated Fair Value - Total | [1] | 129,279 |  | 149,973
Gross Unrealized Losses - Total | [1] | (2,954) |  | (2,330)
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 1,285,907 |  | 1,373,127
Gross Unrealized Losses - Less than 12 Months | [1] | (12,611) |  | (10,089)
Estimated Fair Value - 12 Months or More | [1] | 0 |  | 32,058
Gross Unrealized Losses - 12 Months or More | [1] | 0 |  | (45)
Estimated Fair Value - Total | [1] | 1,285,907 |  | 1,405,185
Gross Unrealized Losses - Total | [1] | (12,611) |  | (10,134)
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 543,844 |  | 1,224,243
Gross Unrealized Losses - Less than 12 Months | [1] | (7,658) |  | (20,163)
Estimated Fair Value - 12 Months or More | [1] | 2,441 |  | 37,610
Gross Unrealized Losses - 12 Months or More | [1] | (402) |  | (167)
Estimated Fair Value - Total | [1] | 546,285 |  | 1,261,853
Gross Unrealized Losses - Total | [1] | (8,060) |  | (20,330)
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months | [1] | 634,470 |  | 441,522
Gross Unrealized Losses - Less than 12 Months | [1] | (9,110) |  | (3,334)
Estimated Fair Value - 12 Months or More | [1] | 57,737 |  | 48,313
Gross Unrealized Losses - 12 Months or More | [1] | (1,605) |  | (1,390)
Estimated Fair Value - Total | [1] | 692,207 |  | 489,835
Gross Unrealized Losses - Total | [1] | (10,715) |  | (4,724)
Short-term investments |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value - Less than 12 Months |  | 97,920 |  | 95,777
Gross Unrealized Losses - Less than 12 Months |  | (2,063) |  | (1,548)
Estimated Fair Value - 12 Months or More |  | 0 |  | 0
Gross Unrealized Losses - 12 Months or More |  | 0 |  | 0
Estimated Fair Value - Total |  | 97,920 |  | 95,777
Gross Unrealized Losses - Total |  | $ (2,063) |  | $ (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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