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Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | Dec. 31, 2016
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 | $ 5,617.8 |  | $ 5,620.1
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 5,781.3 |  | 5,765.4
Available for sale investments in fixed income maturities, Fair Market Value | 5,664.4 |  | 5,664.6
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 | 5,617.8 |  | 5,620.1
Available for sale investments in fixed income maturities, Fair Market Value | 5,664.4 |  | 5,664.6
Non-agency commercial 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 | 11.3 |  | 12.3
Available for sale investments in fixed income maturities, Fair Market Value | $ 11.5 |  | 12.6
Available For Sale Securities Average Ratings By Maturity | AAA | AAA |
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 | $ 1,024.2 |  | 1,062.6
Available for sale investments in fixed income maturities, Fair Market Value | $ 1,032.8 |  | 1,073.9
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 | $ 54.5 |  | 62.6
Available for sale investments in fixed income maturities, Fair Market Value | $ 54.8 |  | 63.0
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 | $ 569.6 |  | 567.2
Due after one year through five years, Cost or Amortized Cost | 2,693.0 |  | 2,643.7
Due after five years through ten years, Cost or Amortized Cost | 1,174.0 |  | 1,172.3
Due after ten years, Cost or Amortized Cost | 91.2 |  | 99.4
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 4,527.8 |  | 4,482.6
Due one year or less, Fair Market Value | 572.1 |  | 570.0
Due after one year through five years, Fair Market Value | 2,721.6 |  | 2,671.9
Due after five years through ten years, Fair Market Value | 1,173.7 |  | 1,168.1
Due after ten years, Fair Market Value | 97.9 |  | 105.1
Available for sale investments in fixed income maturities, Fair Market Value | $ 4,565.3 |  | $ 4,515.1
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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