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Investment Information - Net Realized Gains and Losses (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Net realized gains and losses: |  |  |  |  |  |  |  |  |  |  |  |
Available for sale securities, gross gains on investment sales |  |  |  |  |  |  |  |  |  | $ 595,941 | $ 235,655 | $ 69,299
Available for sale securities, gross losses on investment sales |  |  |  |  |  |  |  |  |  | (117,282) | (104,612) | (223,123)
Equity securities at fair value, net realized gains (losses) on sales during the period | [1] |  |  |  |  |  |  |  |  | 26,849 | 11,313 | (40,117)
Equity securities at fair value, net unrealized gains (losses) on equity securities still held at reporting date | [1] |  |  |  |  |  |  |  |  | 102,394 | 97,768 | (22,828)
Allowance for credit losses - investment related |  |  |  |  |  |  |  |  |  | (3,597) | 0 | 0
Allowance for credit losses - underwriting related |  |  |  |  |  |  |  |  |  | (10,007) | 0 | 0
Net impairment losses |  |  |  |  |  |  |  |  |  | (533) | (3,165) | (2,829)
Derivative instruments | [2] |  |  |  |  |  |  |  |  | 179,675 | 119,741 | 15,636
Other | [3] |  |  |  |  |  |  |  |  | 3,575 | (19,348) | (16,090)
Net realized gains (losses) |  | $ 353,333 | $ 280,499 | $ 556,588 | $ (366,960) | $ 40,830 | $ 61,355 | $ 120,757 | $ 140,256 | 823,460 | 363,198 | (408,173)
Fixed maturities |  |  |  |  |  |  |  |  |  |  |  |
Net realized gains and losses: |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  |  |  |  |  |  |  |  |  | 15,881 | 41,910 | (90,898)
Equity securities |  |  |  |  |  |  |  |  |  |  |  |
Net realized gains and losses: |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  |  |  |  |  |  |  |  |  | 14,629 | 15,869 | (5,984)
Other investments |  |  |  |  |  |  |  |  |  |  |  |
Net realized gains and losses: |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  |  |  |  |  |  |  |  |  | 13,656 | (35,734) | (90,778)
Short-term investments |  |  |  |  |  |  |  |  |  |  |  |
Net realized gains and losses: |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  |  |  |  |  |  |  |  |  | $ 2,279 | $ 3,801 | $ (461)
[1] | Effective January 1, 2018, changes in fair value on equity securities are recorded through net income.
[2] | See Note 11 for information on the Company’s derivative instruments.
[3] | Adoption of ASU 2016-13, “Financial Instruments - Credit Losses (Topic 326)” See note 3.

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