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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 | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Other-than-temporary impairments | $ (0.7) | $ 0.0 | $ 0.0
Catastrophe bonds — net unrealized (losses)/gains | (2.4) | 0.0 | (0.3)
Net change in gross unrealized gains (losses) | 60.3 | 22.5 | (33.1)
Total net realized and unrealized investment gains recorded in the statement of operations | 120.5 | 45.2 | 17.0
Change in available for sale net unrealized gains: |  |  |
Fixed income securities | (14.8) | (39.0) | (82.2)
Short-term investments | 0.0 | 0.0 | (0.1)
Equity securities | 0.0 | 0.0 | (27.3)
Total change in pre-tax available for sale unrealized gains | (14.8) | (39.0) | (109.6)
Change in taxes | 2.0 | 1.3 | 4.4
Total change in net unrealized gains, net of taxes recorded in other comprehensive income | (12.8) | (37.7) | (105.2)
Fixed income securities — Available for sale |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Available for sale securities — gross realized gains | 10.2 | 18.6 | 11.7
Available for sale securities — gross realized (losses) | (6.6) | (8.3) | (2.7)
Cash and cash equivalents — gross realized gains | 0.4 | 0.2 | 0.0
Cash and cash equivalents — gross realized (losses) | (0.1) | (0.6) | 0.0
Equity securities — Available for sale |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Available for sale securities — gross realized gains | 0.0 | 0.0 | 31.9
Available for sale securities — gross realized (losses) | 0.0 | 0.0 | (3.0)
Trading securities — gross realized gains | 59.0 | 54.1 | 46.0
Trading securities — gross realized (losses) | (13.7) | (46.3) | (31.7)
Fixed income securities — Trading |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Cash and cash equivalents — gross realized gains | 1.3 | 0.1 | 0.0
Cash and cash equivalents — gross realized (losses) | 0.0 | (0.3) | 0.0
Trading securities — gross realized gains | 9.7 | 12.6 | 4.9
Trading securities — gross realized (losses) | (4.5) | (7.3) | (6.1)
Short-term Investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Available for sale securities — gross realized gains | 0.1 | 0.0 | 0.0
Trading securities — gross realized gains | 2.7 | 0.0 | 0.0
MVI |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Loss from equity method investment | 0.1 | 0.3 | 0.0
Chaspark |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Loss from equity method investment | (0.9) | (0.3) | 0.6
Bene |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Loss from equity method investment | 0.3 | 0.1 | 0.0
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | AgriLogic |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Gross realized gain on sale of AgriLogic | $ 4.3 | $ 0.0 | $ 0.0

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