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Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading securities, net realized gains | $ 45.5 | $ 1.4 | $ 71.2 | $ (14.7)
Trading securities, Tax on Realized Holding (Losses) Gains on Investments | (7.1) | 9.4 | (8.8) | 8.9
Trading securities realized holding (losses) gain on investments after tax | 38.4 | 10.8 | 62.4 | (5.8)
Foreign Currency Transaction Gains (Losses), Realized | 0.0 | 0.0 | 0.0 | 11.0
Trading Securities, Tax on Realized Foreign Currency Transaction Gains (Losses) Tax | 0.0 | 0.0 | 0.0 | 0.0
Trading securities, realized foreign currency gain (loss) net of tax | 0.0 | 0.0 | 0.0 | 11.0
Net realized investment gains (losses), pre-tax | 45.5 | 1.4 | 71.2 | (3.7)
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (7.1) | 9.4 | (8.8) | 8.9
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 38.4 | 10.8 | 62.4 | 5.2
Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading securities, net realized gains | 3.0 | (8.8) | 2.1 | (22.5)
Foreign Currency Transaction Gains (Losses), Realized | 0.0 | 0.0 | 0.0 | 18.2
Net realized investment gains (losses), pre-tax | 3.0 | (8.8) | 2.1 | (4.3)
Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading securities, net realized gains | 0.1 | (0.7) | 0.1 | (0.8)
Foreign Currency Transaction Gains (Losses), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains (losses), pre-tax | 0.1 | (0.7) | 0.1 | (0.8)
Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading securities, net realized gains | 41.6 | 10.8 | 67.4 | 12.0
Foreign Currency Transaction Gains (Losses), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains (losses), pre-tax | 41.6 | 10.8 | 67.4 | 12.0
Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading securities, net realized gains | 0.8 | 0.1 | 1.6 | (3.4)
Foreign Currency Transaction Gains (Losses), Realized | 0.0 | 0.0 | 0.0 | (7.2)
Net realized investment gains (losses), pre-tax | $ 0.8 | $ 0.1 | $ 1.6 | $ (10.6)

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