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Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Investment securities |  |  |
Fixed maturity investments, at fair value | $ 2,639.7 | $ 2,422.0 |
Sales and maturities of investment securities | 2,162.8 | 3,337.5 | $ 2,672.8
Common Stock |  |  |
Investment securities |  |  |
Trading Securities, Cost | 822.5 | 495.3 |
Net foreign currency gains (losses) | 0.0 | (7.4) |
Fixed maturity investments, at fair value | 1,113.9 | 611.7 |
Trading Securities, Unrealized Holding Gain | 302.8 | 127.4 |
Trading Securities, Unrealized Holding Loss | (11.4) | (3.6) |
Other long-term investments |  |  |
Investment securities |  |  |
Trading Securities, Cost | 304.5 | 286.7 |
Net foreign currency gains (losses) | (2.3) | (1.3) |
Fixed maturity investments, at fair value | 315.8 | 331.9 |
Trading Securities, Unrealized Holding Gain | 32.0 | 53.3 |
Trading Securities, Unrealized Holding Loss | $ (18.4) | $ (6.8) |

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