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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | $ 6.4 | $ 10.2 | $ 18.6
Fixed income securities - gross realizes (losses) | (11.4) | (6.6) | (8.3)
Other-than-temporary impairments | 0.0 | (0.7) | 0.0
Fixed income securities, trading - gross realized gains | 4.6 | 9.7 | 12.6
Fixed income securities, trading, gross realizes (losses) | (25.0) | (4.5) | (7.3)
Equity securities - gross realized gains | 94.5 | 59.0 | 54.1
Equity securities - gross realized (losses) | (20.1) | (13.7) | (46.3)
Catastrophe bonds | 2.2 | (2.4) | 0.0
Net change in gross unrealized (losses) gains | (112.1) | 60.3 | 22.5
Total net realized and unrealized investment (losses)/gains recorded in the statement of operations | (64.7) | 120.5 | 45.2
Change in available for sale net unrealized (losses): |  |  |
Fixed income securities | (81.3) | (14.8) | (39.0)
Change in taxes | 4.8 | 2.0 | 1.3
Total change in net unrealized (losses), net of taxes recorded in other comprehensive income | (76.5) | (12.8) | (37.7)
Short-term investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | 0.0 | 0.1 | 0.0
Fixed income securities - gross realizes (losses) | 0.0 | 0.0 | 0.0
Fixed income securities, trading - gross realized gains | 0.1 | 2.7 | 0.0
Fixed income securities, trading, gross realizes (losses) | (4.2) | 0.0 | 0.0
Cash and cash equivalents |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | 0.3 | 0.4 | 0.2
Fixed income securities - gross realizes (losses) | (0.5) | (0.1) | (0.6)
Fixed income securities, trading - gross realized gains | 1.5 | 1.3 | 0.1
Fixed income securities, trading, gross realizes (losses) | (0.3) | 0.0 | (0.3)
MVI |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | (0.2) | (0.1) | (0.3)
Chaspark |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | 0.0 | 0.9 | 0.3
Digital Re |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | 0.4 | 0.0 | 0.0
Bene |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | (0.9) | (0.3) | (0.1)
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | AgriLogic |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Gross realized gain on sale of AgriLogic | $ 0.0 | $ 4.3 | $ 0.0

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