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Investment Information - Investment in operating affiliates (Details) € / shares in Units, $ / shares in Units, € in Thousands, $ in Thousands |  | 3 Months Ended | 9 Months Ended |  |
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Jul. 01, 2021 USD ($) $ / shares | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2021 EUR (€) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 € / shares | Dec. 31, 2020 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  |  |  | $ 753,916 |  | $ 0 |  |
Investment in operating affiliates |  | $ 1,111,825 |  | 1,111,825 |  |  |  | $ 129,291
Income (loss) from operating affiliates |  | 124,119 | $ 919 | 224,052 |  | $ 6,262 |  |
Net loss on deconsolidation |  | 33,100 |  | 33,100 |  |  |  |
Watford Holdings Ltd | Merger Agreement |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Business acquisition, share price (per share) | $ / shares | $ 35.00 |  |  |  |  |  |  |
Business Acquisition Consideration Paid By Acquirer | $ 278,900 |  |  |  |  |  |  |
Gain on merger transaction | $ 95,700 |  |  |  |  |  |  |
Greysbridge |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Percentage ownership after merger transaction | 40.00% |  |  |  |  |  |  |
Watford Merger Transaction | Watford Holdings Ltd |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net loss on deconsolidation |  | $ 33,100 |  | $ 33,100 |  |  |  |
Kelso And Company | Greysbridge |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by noncontrolling owners | 30.00% |  |  |  |  |  |  |
Warburg Pincus LLC | Greysbridge |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by noncontrolling owners | 30.00% |  |  |  |  |  |  |
Coface |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Percentage ownership |  | 29.50% |  | 29.50% |  |  |  |
Consideration paid per share | € / shares |  |  |  |  |  |  | € 9.95 |
Payments to Acquire Interest in Subsidiaries and Affiliates |  |  |  | $ 546,000 | € 453,000 |  |  |
Investment in operating affiliates |  | $ 615,900 |  | 615,900 |  |  |  |
Income (loss) from operating affiliates |  |  |  | 74,500 |  |  |  |
Purchase Of Affiliate, Deferred Gain |  | $ 36,000 |  | $ 36,000 |  |  |  |
Purchase Of Operating Affiliate, Deferred Gain, Period Of Recognition |  |  |  | 5 years | 5 years |  |  |
Coface | Issued Common Stock |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Percentage ownership |  | 29.86% |  | 29.86% |  |  |  |
Coface | Outstanding Common Stock |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Percentage ownership |  | 30.10% |  | 30.10% |  |  |  |
Greysbridge |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Investment in operating affiliates |  | $ 363,300 |  | $ 363,300 |  |  |  |

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