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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
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Mar. 31, 2018 | Mar. 31, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Other-than-temporary impairments | $ 0.0 | $ (0.3)
Catastrophe bonds | 0.9 | 0.0
Net change in gross unrealized (losses)/gains | (108.8) | 40.9
Total net realized and unrealized investment (losses)/gains recorded in the statement of operations | (37.7) | 46.2
Change in available for sale net unrealized (losses)/gains: |  |
Fixed income securities | (82.9) | 2.0
Total change in pre-tax available for sale unrealized (losses)/gains | (82.9) | 2.0
Change in taxes | 5.5 | 0.1
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax | (77.4) | 2.1
Total fixed income securities — Available for sale |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Fixed income maturities — gross realized gains | 1.8 | 2.2
Fixed income maturities — gross realized (losses) | (1.8) | (1.3)
Cash and cash equivalents - gross realized gains | 0.2 | 0.0
Cash and cash equivalents — gross realized gains (losses) | (0.1) | 0.0
Fixed Income Maturities - Trading |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Cash and cash equivalents - gross realized gains | 1.6 | 0.0
Fixed income securities - gross realized gains | 1.6 | 1.8
Fixed income securities - gross realized (losses) | (7.0) | (2.0)
Equity securities — Trading |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Trading securities, gross realized gain | 94.5 | 4.5
Equity securities — gross realized (losses) | (20.3) | (1.4)
Short-term investments |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Fixed income maturities — gross realized gains | 0.0 | 0.1
MVI |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Change in fair value of investment | (0.1) | 0.0
Chaspark |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Change in fair value of investment | 0.0 | 1.7
Bene |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Change in fair value of investment | $ (0.2) | $ 0.0

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