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Investment Information - Net Investment Income (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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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 investment income: |  |  |  |  |  |  |  |  |  |  |  |
Gross investment income |  |  |  |  |  |  |  |  |  | $ 608,823 | $ 716,643 | $ 655,727
Investment expenses |  |  |  |  |  |  |  |  |  | (89,215) | (88,905) | (92,094)
Net investment income |  | $ 114,458 | $ 128,512 | $ 131,485 | $ 145,153 | $ 154,263 | $ 161,488 | $ 155,038 | $ 156,949 | 519,608 | 627,738 | 563,633
Fixed maturities |  |  |  |  |  |  |  |  |  |  |  |
Net investment income: |  |  |  |  |  |  |  |  |  |  |  |
Gross investment income |  |  |  |  |  |  |  |  |  | 412,481 | 505,399 | 470,912
Term loan investments |  |  |  |  |  |  |  |  |  |  |  |
Net investment income: |  |  |  |  |  |  |  |  |  |  |  |
Gross investment income |  |  |  |  |  |  |  |  |  | 84,149 | 98,949 | 87,926
Equity securities |  |  |  |  |  |  |  |  |  |  |  |
Net investment income: |  |  |  |  |  |  |  |  |  |  |  |
Gross investment income |  |  |  |  |  |  |  |  |  | 28,958 | 15,857 | 13,154
Short-term investments |  |  |  |  |  |  |  |  |  |  |  |
Net investment income: |  |  |  |  |  |  |  |  |  |  |  |
Gross investment income |  |  |  |  |  |  |  |  |  | 10,840 | 15,820 | 18,793
Other |  |  |  |  |  |  |  |  |  |  |  |
Net investment income: |  |  |  |  |  |  |  |  |  |  |  |
Gross investment income | [1] |  |  |  |  |  |  |  |  | $ 72,395 | $ 80,618 | $ 64,942
[1] | Includes income distributions from investment funds and other items.

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