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Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | $ 279.9 | $ 76.9 | $ 166.8
Foreign Currency Transaction Gain (Loss), Realized | 0.4 | 0.4 | 0.0
Net realized investment gains, pre-tax | 280.3 | 77.3 | 166.8
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | (49.5) | (22.8) | (28.4)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax | 0.0 | 0.0 | 0.0
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 49.5 | (22.8) | (28.4)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax | 230.4 | 54.1 | 138.4
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax | 0.4 | 0.4 | 0.0
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 230.8 | 54.5 | 138.4
Fixed maturity investments |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | (2.1) | 1.9 | 5.8
Foreign Currency Transaction Gain (Loss), Realized | 0.4 | 0.0 | 0.0
Net realized investment gains, pre-tax | (1.7) | 1.9 | 5.8
Short-term investments |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 0.4 |  |
Foreign Currency Transaction Gain (Loss), Realized | 0.0 |  |
Net realized investment gains, pre-tax | 0.4 |  |
Common equity securities |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 280.7 | 64.4 | 138.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.4 | 0.0
Net realized investment gains, pre-tax | 280.7 | 64.8 | 138.0
Other long-term investments |  |  |
Investment securities |  |  |
Trading Securities, Realized Gain (Loss) | 0.9 | 10.6 | 23.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0
Net realized investment gains, pre-tax | $ 0.9 | $ 10.6 | $ 23.0

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