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Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | $ 2.7 | $ 42.4 | $ 259.5 | $ 56.7
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | (0.4) | (12.7) | (43.3) | (17.2)
Trading securities realized holding gain (loss) on investments after tax | 2.3 | 29.7 | 216.2 | 39.5
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | (0.1) | 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 | (0.1) | 0.0 | 0.4
Net realized investment gains, pre-tax | 2.7 | 42.3 | 259.5 | 57.1
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (0.4) | (12.7) | (43.3) | (17.2)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 2.3 | 29.6 | 216.2 | 39.9
Fixed maturity investments. |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 1.8 | 1.6 | 0.7 | 2.1
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains, pre-tax | 1.8 | 1.6 | 0.7 | 2.1
Short-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.1 | 0.0 | 0.2 | 0.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains, pre-tax | 0.1 | 0.0 | 0.2 | 0.0
Common equity securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 3.8 | 25.5 | 261.4 | 47.5
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | (0.1) | 0.0 | 0.4
Net realized investment gains, pre-tax | 3.8 | 25.4 | 261.4 | 47.9
Other long-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (3.0) | 15.3 | (2.8) | 7.1
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains, pre-tax | $ (3.0) | $ 15.3 | $ (2.8) | $ 7.1

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