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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 | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income securities, available for sale — gross realized gains | $ 0.8 | $ 2.3 | $ 5.2 | $ 7.8
Fixed income securities, available for sale — gross realized (losses) | (1.1) | (1.6) | (8.5) | (5.3)
Other-than-temporary impairments | 0.0 | (0.1) | 0.0 | (0.5)
Fixed income securities, trading - gross realized gains | 1.0 | 3.0 | 3.6 | 8.3
Fixed income securities, trading - gross realized (losses) | (4.1) | (0.2) | (14.5) | (2.6)
Equity securities — gross realized gains | 0.0 | 46.1 | 94.5 | 55.0
Equity securities — gross realized (losses) | 0.0 | (7.4) | (20.1) | (12.4)
Catastrophe bonds | 0.0 | (3.1) | 0.9 | (3.2)
Net change in gross unrealized gains (losses) | 4.5 | (25.0) | (119.2) | 54.4
Total net realized and unrealized investment (losses) gains recorded in the statement of operations | (0.9) | 17.5 | (59.3) | 105.7
Change in available for sale net unrealized (losses) gains: |  |  |  |
Fixed income securities | (21.0) | 0.3 | (126.7) | 15.2
Total change in pre-tax available for sale unrealized (losses) gains | (21.0) | 0.3 | (126.7) | 15.2
Change in taxes | 1.3 | (0.4) | 8.0 | (1.4)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax | (19.7) | (0.1) | (118.7) | 13.8
Short-term investments |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income securities, available for sale — gross realized gains | 0.0 | 0.0 | 0.0 | 0.1
Fixed income securities, available for sale — gross realized (losses) | (0.1) | 0.0 | (0.1) | 0.0
Fixed income securities, trading - gross realized gains | 0.0 | 1.9 | 0.0 | 2.0
Fixed income securities, trading - gross realized (losses) | (1.4) | 0.0 | (1.4) | 0.0
Cash and cash equivalents |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income securities, available for sale — gross realized gains | 0.0 | 0.3 | 0.2 | 0.3
Fixed income securities, available for sale — gross realized (losses) | 0.0 | 0.0 | (0.4) | (0.1)
Fixed income securities, trading - gross realized gains | 0.0 | 1.3 | 1.5 | 1.3
Fixed income securities, trading - gross realized (losses) | (0.2) | 0.0 | (0.2) | 0.0
MVI |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | 0.0 | 0.0 | (0.2) | (0.1)
Chaspark |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | 0.0 | 0.0 | 0.0 | 0.9
Bene |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | $ (0.3) | $ 0.0 | $ (0.6) | $ (0.2)

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