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Investment Securities (Investment Holdings) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
U.S. Government and agency obligations |  |
Investment securities |  |
Cost or Amortized Cost | $ 231.7 | $ 154.0
Gross Unrealized Gains | 1.0 | 0.1
Gross Unrealized Losses | (0.2) | (0.9)
Fixed maturity investments, at fair value | 232.5 | 153.2
Debt securities issued by corporations |  |
Investment securities |  |
Cost or Amortized Cost | 454.9 | 519.0
Gross Unrealized Gains | 12.5 | 1.0
Gross Unrealized Losses | (0.2) | (9.5)
Fixed maturity investments, at fair value | 467.2 | 510.5
Mortgage and asset-backed securities |  |
Investment securities |  |
Cost or Amortized Cost | 206.6 | 279.0
Gross Unrealized Gains | 2.7 | 2.4
Gross Unrealized Losses | (0.3) | (1.1)
Fixed maturity investments, at fair value | 209.0 | 280.3
Municipal obligations |  |
Investment securities |  |
Cost or Amortized Cost | 284.7 | 136.1
Gross Unrealized Gains | 12.5 | 0.1
Gross Unrealized Losses | (0.1) | (2.7)
Fixed maturity investments, at fair value | 297.1 | 133.5
Total fixed maturity investments |  |
Investment securities |  |
Cost or Amortized Cost | 1,177.9 | 1,088.1
Gross Unrealized Gains | 28.7 | 3.6
Gross Unrealized Losses | (0.8) | (14.2)
Total fixed maturity investments | $ 1,205.8 | $ 1,077.5
Including Short Term Fixed |  |
Investment securities |  |
Duration of fixed maturities (years) | 2 years 9 months 18 days |
Excluding Short Term Investments |  |
Investment securities |  |
Duration of fixed maturities (years) | 3 years 3 months 18 days |

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