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Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 4,126.7 | $ 4,845.2
Available for sale investments in fixed income maturities, Fair Market Value | 4,348.1 | 4,958.2
Total fixed income securities — Available for sale |  |
Debt Securities, Available-for-sale [Line Items] |  |
Due one year or less, Cost or Amortized Cost | 498.6 | 572.7
Due after one year through five years, Cost or Amortized Cost | 1,833.6 | 2,230.3
Due after five years through ten years, Cost or Amortized Cost | 741.9 | 864.1
Due after ten years, Cost or Amortized Cost | 107.6 | 119.0
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 3,181.7 | 3,786.1
Due one year or less, Fair Market Value | $ 501.3 | $ 574.6
Available For Sale Securities Debt Maturities Within One Year Average Ratings By Maturity | AA | AA
Due after one year through five years, Fair Market Value | $ 1,925.9 | $ 2,269.3
Available For Sale Securities Debt Maturities After One Through Five Years Average Ratings By Maturity | AA- | AA-
Due after five years through ten years, Fair Market Value | $ 806.5 | $ 896.3
Available For Sale Securities Debt Maturities After Five Through Ten Years Average Ratings By Maturity | AA- | AA-
Due after ten years, Fair Market Value | $ 131.6 | $ 138.3
Available For Sale Securities Debt Maturities After Ten Years Average Ratings By Maturity | AA- | AA-
Available for sale investments in fixed income maturities, Fair Market Value | $ 3,365.3 | $ 3,878.5
Agency mortgage-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 936.0 | 1,052.2
Cost or Amortized Cost | 936.0 | 1,052.2
Available for sale investments in fixed income maturities, Fair Market Value | $ 973.1 | $ 1,073.0
Available For Sale Securities Average Ratings By Maturity | AA+ | AA+
Non-agency commercial mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 6.7 | $ 6.7
Cost or Amortized Cost | 6.7 | 6.7
Available for sale investments in fixed income maturities, Fair Market Value | $ 7.4 | $ 6.5
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 | $ 2.3 | $ 0.2
Cost or Amortized Cost | 2.3 | 0.2
Available for sale investments in fixed income maturities, Fair Market Value | $ 2.3 | $ 0.2
Available For Sale Securities Average Ratings By Maturity | AAA | AAA
Fixed maturities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost or Amortized Cost | $ 4,126.7 | $ 4,845.2
Available for sale investments in fixed income maturities, Fair Market Value | $ 4,348.1 | $ 4,958.2

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