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Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | $ 68.8 | $ 14.4 | $ 6.4
Fixed income securities - gross realizes (losses) | (1.8) | (7.3) | (11.4)
Net change in expected credit gains (losses) | 0.4 | 0.0 | 0.0
Other-than-temporary impairments |  | 0.0 |
Fixed income securities, trading - gross realized gains | 18.2 | 34.3 | 4.6
Fixed income securities, trading, gross realizes (losses) | (3.8) | (2.6) | (25.0)
Equity securities - gross realized gains | 0.0 | 0.0 | 94.5
Equity securities - gross realized (losses) | 0.0 | 0.0 | (20.1)
Catastrophe bonds | 0.0 | 0.9 | 2.2
Net change in gross unrealized gains / (losses) | 0.0 | 47.2 | (112.1)
Total net realized and unrealized investment gains/(losses) recorded in the statement of operations | 71.1 | 86.2 | (64.7)
Change in available for sale net unrealized gain/(losses): |  |  |
Fixed income securities | 108.5 | 164.9 | (81.3)
Income tax (expense)/benefit | (6.2) | (13.6) | 4.8
Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income | 102.3 | 151.3 | (76.5)
Short-term investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities, trading - gross realized gains | 0.0 | 0.0 | 0.1
Fixed income securities, trading, gross realizes (losses) | 0.0 | 0.0 | (4.2)
Cash and cash equivalents |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | 0.5 | 0.1 | 0.3
Fixed income securities - gross realizes (losses) | (0.4) | (0.2) | (0.5)
Fixed income securities, trading - gross realized gains | 0.2 | 0.0 | 1.5
Fixed income securities, trading, gross realizes (losses) | (0.3) | (0.3) | (0.3)
Privately-held investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities, trading - gross realized gains | 0.0 | 0.2 | 0.0
Fixed income securities, trading, gross realizes (losses) | 0.0 | (0.2) | 0.0
Net change in gross unrealized gains / (losses) | 20.4 | 0.0 | 0.0
MVI |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | (0.4) | (0.1) | (0.2)
Bene |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | 0.0 | 0.0 | (0.9)
Digital Re |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | (0.3) | (0.2) | 0.4
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Crop Re |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Gross realized gain on sale of equity method investment | 8.6 | 0.0 | 0.0
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Bene |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Gross realized gain on sale of equity method investment | $ 1.8 | $ 0.0 | $ 0.0

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