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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, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Investment securities |  |  |
Sales and maturities of investment securities | $ 1,000.0 | $ 2,200.0 | $ 2,800.0
Common Stock |  |  |
Investment securities |  |  |
Cost or Amortized Cost | 553.3 | 904.7 |
Gross Unrealized Gains | 130.6 | 51.0 |
Gross Unrealized Losses | 0.0 | (30.1) |
Net Foreign Currency Gains | 0.0 | 0.0 |
Fixed maturity investments, at fair value | 683.9 | 925.6 |
Other long-term investments |  |  |
Investment securities |  |  |
Cost or Amortized Cost | 667.4 | 330.3 |
Gross Unrealized Gains | 255.2 | 52.2 |
Gross Unrealized Losses | (64.1) | (54.9) |
Net Foreign Currency Gains | (2.2) | (2.0) |
Fixed maturity investments, at fair value | $ 856.3 | $ 325.6 |

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