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Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items] |  |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 5,465.2 |  | $ 5,201.2
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 5,519.9 |  | 5,291.2
Available for sale investments in fixed income maturities, Fair Market Value | 5,412.2 |  | 5,231.0
Total fixed income securities — Available for sale |  |  |
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items] |  |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 5,465.2 |  | 5,201.2
Available for sale investments in fixed income maturities, Fair Market Value | 5,412.2 |  | 5,231.0
Agency mortgage-backed |  |  |
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items] |  |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 985.6 |  | 941.0
Available for sale investments in fixed income maturities, Fair Market Value | $ 973.8 |  | 946.5
Available For Sale Securities Average Ratings By Maturity | AA+ | AA+ |
Asset-backed |  |  |
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items] |  |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 18.7 |  | 26.3
Available for sale investments in fixed income maturities, Fair Market Value | $ 18.5 |  | 26.2
Available For Sale Securities Average Ratings By Maturity | AAA | AAA |
Total fixed income securities — Available for sale |  |  |
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items] |  |  |
Due one year or less, Cost or Amortized Cost | $ 592.2 |  | 561.7
Due after one year through five years, Cost or Amortized Cost | 2,608.0 |  | 2,486.7
Due after five years through ten years, Cost or Amortized Cost | 1,153.4 |  | 1,092.2
Due after ten years, Cost or Amortized Cost | 107.3 |  | 93.3
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 4,460.9 |  | 4,233.9
Due one year or less, Fair Market Value | 593.2 |  | 562.4
Due after one year through five years, Fair Market Value | 2,582.6 |  | 2,492.2
Due after five years through ten years, Fair Market Value | 1,128.1 |  | 1,097.4
Due after ten years, Fair Market Value | 116.0 |  | 106.3
Available for sale investments in fixed income maturities, Fair Market Value | $ 4,419.9 |  | $ 4,258.3
Available For Sale Securities Average for one year or less | AA | AA |
Available For Sale Securities Average after one year through five years | AA- | AA- |
Available For Sale Securities Average after five years through ten years | AA- | A+ |
Available For Sale Securities Debt Maturities After Ten Years Average Ratings By Maturity | A+ | A |

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