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Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Debt and Equity Securities, FV-NI [Line Items] |  |
Unrealized gains (losses) on investments | $ 220.2 | $ (37.9)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments | (50.5) | 6.3
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax | 169.7 | (31.6)
Foreign Currency Transaction Gain (Loss), Unrealized | (0.3) | (10.7)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax | 0.0 | 0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax | (0.3) | (10.7)
Net unrealized investment gains (losses), pre-tax (1) | 219.9 | (48.6)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax | (50.5) | 6.3
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 169.4 | (42.3)
Fixed maturity investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Unrealized gains (losses) on investments | 22.0 | (18.6)
Foreign Currency Transaction Gain (Loss), Unrealized | 0.0 | (14.8)
Net unrealized investment gains (losses), pre-tax (1) | 22.0 | (33.4)
Short-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Unrealized gains (losses) on investments | 0.0 | (0.7)
Foreign Currency Transaction Gain (Loss), Unrealized | 0.0 | 0.0
Net unrealized investment gains (losses), pre-tax (1) | 0.0 | (0.7)
Common equity securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Unrealized gains (losses) on investments | 85.5 | (16.7)
Foreign Currency Transaction Gain (Loss), Unrealized | 0.0 | 0.0
Net unrealized investment gains (losses), pre-tax (1) | 85.5 | (16.7)
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Unrealized gains (losses) on investments | 112.7 | (1.9)
Foreign Currency Transaction Gain (Loss), Unrealized | (0.3) | 4.1
Net unrealized investment gains (losses), pre-tax (1) | 112.4 | $ 2.2
MediaAlpha |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Unrealized investment gain | $ 114.7 |

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