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Investment Information - Net realized gains and losses (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Available for sale securities: |  |  |  |  |
Available for sale securities, gross gains on investment sales |  | $ 115,541 | $ 232,153 | $ 180,543 | $ 410,353
Available for sale securities, gross losses on investment sales |  | (50,627) | (49,824) | (113,625) | (81,792)
Equity securities, at fair value: |  |  |  |  |
Net realized gains (losses) on sales during the period |  | 33,570 | (18,250) | 71,419 | (18,789)
Net unrealized gains (losses) on equity securities still held at reporting date |  | 65,847 | 145,686 | 85,555 | (29,880)
Allowance for credit losses: |  |  |  |  |
Allowance for credit losses - investment related |  | (59) | (1,000) | (2,662) | (10,349)
Allowance for credit losses - underwriting related |  | 1,381 | (5,834) | 6,649 | (9,104)
Net impairment losses |  |  |  | 0 | (533)
Derivative instruments | [1] | (51,109) | (836) | (14,993) | 126,353
Other |  | 10,224 | (6,515) | (12,821) | (8,612)
Net realized gains (losses) |  | 202,907 | 556,588 | 345,368 | 189,628
Fixed maturities |  |  |  |  |
Available for sale securities: |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 10,912 | 68,181 | 27,465 | (59,485)
Allowance for credit losses: |  |  |  |  |
Allowance for credit losses - investment related |  | 896 | 3,225 | (752) | (6,095)
Other investments |  |  |  |  |
Available for sale securities: |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 60,884 | 178,570 | 107,739 | (129,230)
Equity securities |  |  |  |  |
Available for sale securities: |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | 5,492 | 6,664 | 7,557 | 1,755
Short-term investments |  |  |  |  |
Available for sale securities: |  |  |  |  |
Change in fair value of assets and liabilities accounted for using the fair value option |  | $ (104) | $ 3,368 | $ 632 | $ (5,313)
[1] | See note 9 for information on the Company’s derivative instruments.

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