Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2017	Sep. 30, 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	$ 5,305.7		$ 5,620.1
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,340.1		5,765.4
Available for sale investments in fixed income maturities, Fair Market Value	5,365.4		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	5,305.7		5,620.1
Available for sale investments in fixed income maturities, Fair Market Value	5,365.4		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			12.3
Available for sale investments in fixed income maturities, Fair Market Value			12.6
Available For Sale Securities Average Ratings By Maturity		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	940.8		1,062.6
Available for sale investments in fixed income maturities, Fair Market Value	$ 952.8		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	$ 33.6		62.6
Available for sale investments in fixed income maturities, Fair Market Value	$ 33.7		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	$ 577.8		567.2
Due after one year through five years, Cost or Amortized Cost	2,597.3		2,643.7
Due after five years through ten years, Cost or Amortized Cost	1,060.6		1,172.3
Due after ten years, Cost or Amortized Cost	95.6		99.4
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,331.3		4,482.6
Due one year or less, Fair Market Value	579.2		570.0
Due after one year through five years, Fair Market Value	2,623.2		2,671.9
Due after five years through ten years, Fair Market Value	1,070.1		1,168.1
Due after ten years, Fair Market Value	106.4		105.1
Available for sale investments in fixed income maturities, Fair Market Value	$ 4,378.9		$ 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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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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