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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment securities |  |  |
Fixed maturity investments, at fair value |  | $ 2,081.1 |
Sales and maturities of investment securities | $ 2,791.3 | 3,692.4 | $ 750.7
Common Stock |  |  |
Investment securities |  |  |
Cost or amortized cost | 739.7 | 258.6 |
Gross unrealized gains | 129.4 | 29.0 |
Gross unrealized losses | (3.0) | (2.0) |
Net foreign currency gains | 0.0 | 0.0 |
Fixed maturity investments, at fair value | 866.1 | 285.6 |
Other long-term investments |  |  |
Investment securities |  |  |
Cost or amortized cost | 246.6 | 194.0 |
Gross unrealized gains | 6.8 | 7.9 |
Gross unrealized losses | (39.7) | (25.2) |
Net foreign currency gains | (4.9) | (3.9) |
Fixed maturity investments, at fair value | $ 208.8 | $ 172.8 |

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