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Investment Information - Net realized gains and losses (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Available for sale securities: |  |  |  |  |
Available for sale securities, gross gains on investment sales |  | $ 104,733 | $ 73,685 | $ 515,086 | $ 192,140
Available for sale securities, gross losses on investment sales |  | (16,862) | (30,561) | (98,654) | (77,498)
Equity securities, at fair value: |  |  |  |  |
Net realized gains (losses) on sales during the period |  | 26,549 | 5,217 | 7,760 | 9,503
Net unrealized gains (losses) on equity securities still held at reporting date |  | 33,562 | (1,206) | 3,682 | 58,562
Allowance for credit losses: |  |  |  |  |
Allowance for credit losses - investment related |  | (229) |  | (10,578) |
Allowance for credit losses - underwriting related |  | 351 | 0 | (8,753) | 0
Net impairment losses |  | 0 | (1,163) | (533) | (2,521)
Derivative instruments | [1] | 20,369 | 42,893 | 146,722 | 142,730
Other |  | 7,303 | 1,335 | (1,309) | (8,703)
Net realized gains (losses) |  | 280,499 | 61,355 | 470,127 | 322,368
Fixed maturities |  |  |  |  |
Available for sale securities: |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 34,115 | (3,895) | (25,370) | 38,682
Allowance for credit losses: |  |  |  |  |
Allowance for credit losses - investment related |  | 1,332 | 0 | (4,763) | 0
Other investments |  |  |  |  |
Available for sale securities: |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 61,622 | (21,778) | (67,608) | (37,363)
Equity securities |  |  |  |  |
Available for sale securities: |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 4,048 | (1,231) | 5,803 | 9,449
Short-term investments |  |  |  |  |
Available for sale securities: |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | $ 3,377 | $ (1,941) | $ (1,936) | $ (2,613)
[1] | See note 9 for information on the Company’s derivative instruments.

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