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Investment Information - Summary of Available for Sale Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Available for sale securities: |  |  |  |
Estimated Fair Value |  | $ 20,921,530 |  | $ 18,219,582
Gross Unrealized Gains |  | 657,620 |  | 351,114
Gross Unrealized Losses |  | (79,654) |  | (55,003)
Cost or Amortized Cost |  | 18,143,305 |  | 16,598,808
Short-term investments available for sale, at amortized cost |  | 1,924,292 |  | 957,283
Available for Sale Investments Cost Or Amortized Cost Fixed Maturities And Short Term |  | 20,345,961 |  | 17,923,471
Fixed maturities and fixed maturities pledged under securities lending agreements |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 18,996,608 |  | 17,263,036
Gross Unrealized Gains |  | 654,927 |  | 350,303
Gross Unrealized Losses |  | (77,591) |  | (53,455)
Allowance for Expected Credit Losses |  | (2,397) |  | 0
Cost or Amortized Cost |  | 18,421,669 |  | 16,966,188
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 7,856,571 |  | 6,406,591
Gross Unrealized Gains | [1] | 414,247 |  | 191,889
Gross Unrealized Losses | [1] | (34,388) |  | (12,793)
Allowance for Expected Credit Losses | [2] | (896) |  |
Cost or Amortized Cost |  | 7,477,608 |  | 6,227,495
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 630,001 |  | 562,309
Gross Unrealized Gains | [1] | 8,939 |  | 9,669
Gross Unrealized Losses | [1] | (5,028) |  | (931)
Allowance for Expected Credit Losses | [2] | (278) |  |
Cost or Amortized Cost |  | 626,368 |  | 553,571
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 494,522 |  | 881,926
Gross Unrealized Gains | [1] | 27,291 |  | 24,628
Gross Unrealized Losses | [1] | (3,835) |  | (2,213)
Allowance for Expected Credit Losses |  | (11) | [2] | 0
Cost or Amortized Cost |  | 471,077 |  | 859,511
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 389,900 |  | 733,108
Gross Unrealized Gains | [1] | 8,722 |  | 14,951
Gross Unrealized Losses | [1] | (2,954) |  | (2,330)
Allowance for Expected Credit Losses | [2] | (122) |  |
Cost or Amortized Cost |  | 384,254 |  | 720,487
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 5,557,077 |  | 4,916,592
Gross Unrealized Gains | [1] | 22,612 |  | 36,600
Gross Unrealized Losses | [1] | (12,611) |  | (10,134)
Allowance for Expected Credit Losses | [2] | 0 |  |
Cost or Amortized Cost |  | 5,547,076 |  | 4,890,126
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 2,433,733 |  | 2,078,757
Gross Unrealized Gains | [1] | 153,891 |  | 48,549
Gross Unrealized Losses | [1] | (8,060) |  | (20,330)
Allowance for Expected Credit Losses | [2] | 0 |  |
Cost or Amortized Cost |  | 2,287,902 |  | 2,050,538
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 1,634,804 |  | 1,683,753
Gross Unrealized Gains | [1] | 19,225 |  | 24,017
Gross Unrealized Losses | [1] | (10,715) |  | (4,724)
Allowance for Expected Credit Losses | [2] | (1,090) |  |
Cost or Amortized Cost |  | 1,627,384 |  | 1,664,460
Short-term investments |  |  |  |
Available for sale securities: |  |  |  |
Estimated Fair Value |  | 1,924,922 |  | 956,546
Gross Unrealized Gains |  | 2,693 |  | 811
Gross Unrealized Losses |  | (2,063) |  | (1,548)
Allowance for Expected Credit Losses |  | 0 |  |
Short-term investments available for sale, at amortized cost |  | $ 1,924,292 |  | $ 957,283
[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.”
[2] | Effective January 1, 2020, the Company adopted ASU 2016-13 and as a result any credit impairment losses on the Company’s available-for-sale investments are recorded as an allowance, subject to reversal.

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