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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, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
OTTI charge recognized | $ (0.1) | $ 0.0 | $ (0.4) | $ 0.0
Catastrophe bonds | (0.1) | 0.0 | (0.1) | (0.2)
Net change in gross unrealized gains | 38.5 | 29.4 | 79.4 | 80.8
Total net realized and unrealized investment gains recorded in the statement of operations | 42.0 | 36.8 | 88.2 | 81.8
Change in available for sale net unrealized gains: |  |  |  |
Fixed income securities | 12.9 | 42.2 | 14.9 | 127.2
Total change in pre-tax available for sale unrealized gains | 12.9 | 42.2 | 14.9 | 127.2
Change in taxes | (1.1) | (5.0) | (1.0) | (13.1)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax | 11.8 | 37.2 | 13.9 | 114.1
Total fixed income securities — Available for sale |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income maturities — gross realized gains | 3.3 | 2.1 | 5.5 | 9.2
Fixed income maturities — gross realized (losses) | (2.4) | (1.3) | (3.7) | (3.7)
Cash and Cash Equivalents Gross Realized Gains | 0.0 | 0.1 | 0.0 | 0.1
Cash and cash equivalents — gross realized gains (losses) | (0.1) | 0.2 | (0.1) | (0.5)
Fixed Income Maturities - Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income maturities — gross realized gains | 3.5 | 4.0 | 5.3 | 5.2
Fixed income maturities — gross realized (losses) | (0.4) | (0.6) | (2.4) | (6.4)
Equity securities — Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Trading securities, gross realized gain | 4.4 | 9.3 | 8.9 | 15.2
Equity securities — gross realized (losses) | (3.6) | (6.5) | (5.0) | (18.0)
Short-term investments |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Fixed income maturities — gross realized gains | 0.0 | 0.2 | 0.1 | 0.2
Fixed income maturities — gross realized (losses) | 0.0 | (0.1) | 0.0 | (0.1)
Trading securities, gross realized gain | 0.1 | 0.0 | 0.1 | 0.0
MVI |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | (0.1) | 0.0 | (0.1) | 0.0
Chaspark |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | (0.8) | 0.0 | 0.9 | 0.0
Bene |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Change in fair value of investment | $ (0.2) | $ 0.0 | $ (0.2) | $ 0.0

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