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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 | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Other-than-temporary impairments | $ 0.0 | $ (0.1) | $ 0.0 | $ (0.4)
Catastrophe bonds | 0.0 | (0.1) | 0.9 | (0.1)
Net change in gross unrealized (losses) gains | (14.9) | 38.5 | (123.7) | 79.4
Total net realized and unrealized investment (losses) gains recorded in the statement of operations | (20.7) | 42.0 | (58.4) | 88.2
Change in available for sale net unrealized (losses) gains: |  |  |  |
Fixed income securities | (22.8) | 12.9 | (105.7) | 14.9
Total change in pre-tax available for sale unrealized (losses) gains | (22.8) | 12.9 | (105.7) | 14.9
Change in taxes | 1.2 | (1.1) | 6.7 | (1.0)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax | (21.6) | 11.8 | (99.0) | 13.9
Total fixed income securities — Available for sale |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income maturities — gross realized gains | 2.6 | 3.3 | 4.4 | 5.5
Fixed income maturities — gross realized (losses) | (5.6) | (2.4) | (7.4) | (3.7)
Cash and cash equivalents - gross realized gains | 0.0 | 0.0 | 0.2 | 0.0
Cash and cash equivalents — gross realized gains (losses) | (0.3) | (0.1) | (0.4) | (0.1)
Fixed Income Maturities - Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Cash and cash equivalents — gross realized gains (losses) | (0.1) | 0.0 | 1.5 | 0.0
Fixed income securities - gross realized gains | 1.0 | 3.5 | 2.6 | 5.3
Fixed income securities - gross realized (losses) | (3.4) | (0.4) | (10.4) | (2.4)
Equity securities — Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Trading securities, gross realized gain | 0.0 | 4.4 | 94.5 | 8.9
Equity securities — gross realized (losses) | 0.2 | (3.6) | (20.1) | (5.0)
Short-term investments |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income maturities — gross realized gains | 0.0 | 0.0 | 0.0 | 0.1
Trading securities, gross realized gain | 0.0 | 0.1 | 0.0 | 0.1
MVI |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | (0.1) | (0.1) | (0.2) | (0.1)
Chaspark |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | 0.0 | (0.8) | 0.0 | 0.9
Bene |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | $ (0.1) | $ (0.2) | $ (0.3) | $ (0.2)

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