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Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items] |  |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 5,393.8 |  | $ 5,291.2
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 5,273.4 |  | 5,201.2
Available for sale investments in fixed income maturities, Fair Market Value | 5,296.8 |  | 5,320.9
Total fixed income securities — Available for sale |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 5,273.4 |  | 5,201.2
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 5,273.4 |  | 5,201.2
Available for sale investments in fixed income maturities, Fair Market Value | 5,176.4 |  | 5,231.0
Agency mortgage-backed |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 934.2 |  | 941.0
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 934.2 |  | 941.0
Available for sale investments in fixed income maturities, Fair Market Value | $ 909.4 |  | 946.5
Available For Sale Securities Average Ratings By Maturity | AA+ | AA+ |
Asset-backed |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 18.7 |  | 26.3
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 |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Due one year or less, Cost or Amortized Cost | $ 445.6 |  | 561.7
Due after one year through five years, Cost or Amortized Cost | 2,720.5 |  | 2,486.7
Due after five years through ten years, Cost or Amortized Cost | 1,044.4 |  | 1,092.2
Due after ten years, Cost or Amortized Cost | 110.0 |  | 93.3
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 4,320.5 |  | 4,233.9
Due one year or less, Fair Market Value | 445.5 |  | 562.4
Due after one year through five years, Fair Market Value | 2,680.5 |  | 2,492.2
Due after five years through ten years, Fair Market Value | 1,008.5 |  | 1,097.4
Due after ten years, Fair Market Value | 114.0 |  | 106.3
Available for sale investments in fixed income maturities, Fair Market Value | $ 4,248.5 |  | $ 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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