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Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | $ 8.0 | $ (7.4) | $ 267.5 | $ 49.3
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | (2.7) | 1.5 | (46.0) | (15.7)
Trading securities realized holding gain (loss) on investments after tax | 5.3 | (5.9) | 221.5 | 33.6
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.4
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 | 0.0 | 0.0 | 0.0 | 0.4
Net realized investment gains (losses), pre-tax | 8.0 | (7.4) | 267.5 | 49.7
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (2.7) | 1.5 | (46.0) | (15.7)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 5.3 | (5.9) | 221.5 | 34.0
Fixed maturity investments. |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 2.8 | (0.1) | 3.5 | 2.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains (losses), pre-tax | 2.8 | (0.1) | 3.5 | 2.0
Short-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.2 | 0.0 | 0.4 | 0.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains (losses), pre-tax | 0.2 | 0.0 | 0.4 | 0.0
Common equity securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 6.4 | (5.9) | 267.7 | 41.6
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.4
Net realized investment gains (losses), pre-tax | 6.4 | (5.9) | 267.7 | 42.0
Other long-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (1.4) | (1.4) | (4.1) | 5.7
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains (losses), pre-tax | $ (1.4) | $ (1.4) | $ (4.1) | $ 5.7

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