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Investment Information - Narrative (Details) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($) lots | Dec. 31, 2019 USD ($) lots | Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items] |  |  |
Investable assets | $ 29,513,907 | $ 24,990,265 | $ 22,324,524
Continuous unrealized loss, qualitative disclosures: |  |  |
Number of positions in an unrealized loss position (lots) | lots | 2,320 | 2,230 |
Total number of positions (lots) | lots | 11,180 | 9,590 |
Largest single loss | $ 900 | $ 900 |
Equity securities, at fair value | 1,444,830 | 838,925 |
Equity Method Investment, Financial Statement, Reported Amounts [Abstract] |  |  |
Equity in net income (loss) of investment funds accounted for using the equity method | $ 146,693 | $ 123,672 | $ 45,641
Equity method investments, time lag for reporting - low end of range | one |  |
Equity method investments, time lag for reporting - high end of range | three |  |
Watford Holdings Ltd |  |  |
Segment Reporting Information [Line Items] |  |  |
Investable assets | $ 2,700,000 |  |
ACGL |  |  |
Segment Reporting Information [Line Items] |  |  |
Investable assets | $ 26,900,000 |  |

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