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Investments - Other Investments (Details) - USD ($) $ in Millions |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 14, 2020 | Jul. 26, 2016 | Jan. 31, 2015 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  |  |  | $ 67.9 | $ 67.1 |  |
Investment in the period |  |  |  | 0.5 | 1.1 |  |
Unrealized gain/(loss) for the period |  |  |  | (0.7) | (0.3) |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 10.4 |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  | (77.2) |  |  |
Closing value of investment, ending balance |  |  |  | 0.9 | 67.9 | $ 67.1 |
MVI |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  |  |  | 0.4 | 0.5 |  |
Investment in the period |  |  | $ 0.8 | 0.0 | 0.0 | 0.2 | $ 0.1
Unrealized gain/(loss) for the period |  |  |  | (0.4) | (0.1) |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 0.0 |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  | 0.0 |  |  |
Closing value of investment, ending balance |  |  |  | 0.0 | 0.4 | 0.5 |
Bene |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  |  |  | 4.3 | 3.2 |  |
Investment in the period |  | $ 3.3 |  |  |  |  |
Equity method investment, cash payment |  |  |  | 0.0 | 1.1 | 1.2 |
Unrealized gain/(loss) for the period |  |  |  | 0.0 | 0.0 |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 1.8 |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  | (6.1) |  |  |
Closing value of investment, ending balance |  |  |  | 0.0 | 4.3 | 3.2 |
Digital Re |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  |  |  | 0.7 | 0.9 |  |
Investment in the period |  |  |  | 0.0 | 0.0 |  |
Unrealized gain/(loss) for the period |  |  |  | (0.3) | (0.2) |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 0.0 |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  | 0.0 |  |  |
Closing value of investment, ending balance |  |  |  | 0.4 | 0.7 | 0.9 |
Crop Re |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  |  |  | 62.5 | 62.5 |  |
Investment in the period |  |  |  | 0.0 | 0.0 |  |
Unrealized gain/(loss) for the period |  |  |  | 0.0 | 0.0 |  |
Equity method investment, realized gain (loss) on disposal | $ 71.1 |  |  | 8.6 |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  | (71.1) |  |  |
Closing value of investment, ending balance |  |  |  | 0.0 | 62.5 | 62.5 |
Multi-Line Insurer |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  |  |  | 0.0 | 0.0 |  |
Investment in the period |  |  |  | 0.5 | 0.0 |  |
Unrealized gain/(loss) for the period |  |  |  | 0.0 | 0.0 |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 0.0 |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  | 0.0 |  |  |
Closing value of investment, ending balance |  |  |  | $ 0.5 | $ 0.0 | $ 0.0 |

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