Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$ 5,177.2		$ 5,387.9
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,016.5		5,282.3
Available for sale investments in fixed income maturities, Fair Market Value	5,201.4		5,336.3
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	5,016.5		5,282.3
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,016.5		5,282.3
Available for sale investments in fixed income maturities, Fair Market Value	5,040.5		5,230.7
Agency mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	955.3		1,025.1
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	955.3		1,025.1
Available for sale investments in fixed income maturities, Fair Market Value	$ 955.0		1,012.6
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	$ 16.2		17.4
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	16.2		17.4
Available for sale investments in fixed income maturities, Fair Market Value	$ 16.1		17.3
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	$ 441.1		464.3
Due after one year through five years, Cost or Amortized Cost	2,463.8		2,605.7
Due after five years through ten years, Cost or Amortized Cost	1,014.7		1,047.9
Due after ten years, Cost or Amortized Cost	125.4		121.9
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,045.0		4,239.8
Due one year or less, Fair Market Value	440.7		463.5
Due after one year through five years, Fair Market Value	2,467.0		2,582.0
Due after five years through ten years, Fair Market Value	1,025.1		1,028.3
Due after ten years, Fair Market Value	136.6		127.0
Available for sale investments in fixed income maturities, Fair Market Value	$ 4,069.4		$ 4,200.8
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-	AA-
Available For Sale Securities Debt Maturities After Ten Years Average Ratings By Maturity	AA-	AA-

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