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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