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Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | $ 13.3 | $ 1.6 | $ 13.7 | $ 264.3
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | (2.7) | (0.1) | (2.9) | (44.9)
Trading securities realized holding gain (loss) on investments after tax | 10.6 | 1.5 | 10.8 | 219.4
Foreign Currency Transaction Gain (Loss), Realized | 0.1 | 0.0 | 0.3 | 0.0
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax | 0.0 | 0.0 | 0.0 | 0.0
Trading securities, realized foreign currency gain (loss) net of tax | 0.1 | 0.0 | 0.3 | 0.0
Net realized investment gains, pre-tax | 13.4 | 1.6 | 14.0 | 264.3
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (2.7) | (0.1) | (2.9) | (44.9)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 10.7 | 1.5 | 11.1 | 219.4
Fixed maturity investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.1 | 1.4 | (1.0) | 1.7
Foreign Currency Transaction Gain (Loss), Realized | 1.3 | 0.0 | 1.4 | 0.0
Net realized investment gains, pre-tax | 1.4 | 1.4 | 0.4 | 1.7
Short-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.0 | 0.1 | 0.0 | 0.2
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains, pre-tax | 0.0 | 0.1 | 0.0 | 0.2
Common equity securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 12.8 | 0.1 | 13.6 | 262.4
Foreign Currency Transaction Gain (Loss), Realized | 0.5 | 0.0 | 0.6 | 0.0
Net realized investment gains, pre-tax | 13.3 | 0.1 | 14.2 | 262.4
Other long-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.4 | 0.0 | 1.1 | 0.0
Foreign Currency Transaction Gain (Loss), Realized | (1.7) | 0.0 | (1.7) | 0.0
Net realized investment gains, pre-tax | $ (1.3) | $ 0.0 | $ (0.6) | $ 0.0

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