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Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2016 | Mar. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | $ 256.8 | $ 14.3
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | (42.9) | (4.5)
Trading securities realized holding gain (loss) on investments after tax | 213.9 | 9.8
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.5
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax | 0.0 | 0.0
Trading securities, realized foreign currency gain (loss) net of tax | 0.0 | 0.5
Net realized investment gains, pre-tax | 256.8 | 14.8
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (42.9) | (4.5)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 213.9 | 10.3
Fixed maturity investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | (1.1) | 0.5
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0
Net realized investment gains, pre-tax | (1.1) | 0.5
Short-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | 0.0 | 0.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0
Net realized investment gains, pre-tax | 0.0 | 0.0
Common equity securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | 257.6 | 22.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.5
Net realized investment gains, pre-tax | 257.6 | 22.5
Other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Securities, Realized Gain (Loss) | 0.3 | (8.2)
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0
Net realized investment gains, pre-tax | $ 0.3 | $ (8.2)

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