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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Other-than-temporary impairments | $ (0.1) | $ 0.0 | $ (0.5) | $ 0.0
Catastrophe bonds | (3.1) | 0.3 | (3.2) | 0.1
Net change in gross unrealized (losses)/gains | (25.0) | 8.0 | 54.4 | 88.8
Total net realized and unrealized investment gains recorded in the statement of operations | 17.5 | 21.5 | 105.7 | 103.3
Change in available for sale net unrealized gains: |  |  |  |
Fixed income securities | 0.3 | (23.2) | 15.2 | 104.0
Total change in pre-tax available for sale unrealized gains | 0.3 | (23.2) | 15.2 | 104.0
Change in taxes | (0.4) | 2.0 | (1.4) | (11.1)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax | (0.1) | (21.2) | 13.8 | 92.9
Total fixed income securities — Available for sale |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income maturities — gross realized gains | 2.3 | 5.5 | 7.8 | 14.7
Fixed income maturities — gross realized (losses) | (1.6) | (0.9) | (5.3) | (4.6)
Cash and cash equivalents - gross realized gains | 0.3 | 0.0 | 0.3 | 0.1
Cash and cash equivalents — gross realized gains (losses) | 0.0 | 0.0 | (0.1) | (0.5)
Fixed Income Maturities - Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Cash and cash equivalents - gross realized gains | 1.3 | 0.0 | 1.3 | 0.0
Fixed income securities - gross realized gains | 3.0 | 4.3 | 8.3 | 9.5
Fixed income securities - gross realized (losses) | (0.2) | (0.5) | (2.6) | (6.9)
Equity securities — Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Trading securities, gross realized gain | 46.1 | 8.1 | 55.0 | 23.3
Equity securities — gross realized (losses) | (7.4) | (4.2) | (12.4) | (22.2)
Short-term investments |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income maturities — gross realized gains | 0.0 | 0.1 | 0.1 | 0.0
Fixed income maturities — gross realized (losses) | 0.0 | (0.2) | 0.0 | 0.0
Trading securities, gross realized gain | 1.9 | 0.0 | 2.0 | 0.0
MVI |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | 0.0 | 0.0 | (0.1) | 0.0
Chaspark |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | 0.0 | 1.0 | 0.9 | 1.0
Bene |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | $ 0.0 | $ 0.0 | $ (0.2) | $ 0.0

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