Investments (Investment Category and Duration of Gross Unrealized Losses on Fixed Maturity Securities and Equity Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	$ 1,308.6	$ 1,755.1
Gross unrealized losses on securities, less than 12 months, unrealized losses	(19.7)	(29.2)
Gross unrealized losses on securities, 12 months or more, fair value	85.6	44.9
Gross unrealized losses on securities, 12 months or more, unrealized losses	(2.2)	(3.7)
Gross unrealized losses on securities, fair value, total	1,394.2	1,800.0
Gross unrealized losses on securities, unrealized losses, total	(21.9)	(32.9)
Fixed maturity securities | U.S. government and government agencies and authorities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	99.6	91.0
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.5)	(1.3)
Gross unrealized losses on securities, 12 months or more, fair value	4.7	0.0
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.1)	0.0
Gross unrealized losses on securities, fair value, total	104.3	91.0
Gross unrealized losses on securities, unrealized losses, total	(0.6)	(1.3)
Fixed maturity securities | States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	0.0	16.9
Gross unrealized losses on securities, less than 12 months, unrealized losses	0.0	(0.4)
Gross unrealized losses on securities, 12 months or more, fair value	2.4	0.0
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.1)	0.0
Gross unrealized losses on securities, fair value, total	2.4	16.9
Gross unrealized losses on securities, unrealized losses, total	(0.1)	(0.4)
Fixed maturity securities | Foreign governments
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	49.0	98.8
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.8)	(0.9)
Gross unrealized losses on securities, 12 months or more, fair value	0.0	0.0
Gross unrealized losses on securities, 12 months or more, unrealized losses	0.0	0.0
Gross unrealized losses on securities, fair value, total	49.0	98.8
Gross unrealized losses on securities, unrealized losses, total	(0.8)	(0.9)
Fixed maturity securities | Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	26.3	0.0
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.2)	0.0
Gross unrealized losses on securities, 12 months or more, fair value	0.0	1.0
Gross unrealized losses on securities, 12 months or more, unrealized losses	0.0	(0.1)
Gross unrealized losses on securities, fair value, total	26.3	1.0
Gross unrealized losses on securities, unrealized losses, total	(0.2)	(0.1)
Fixed maturity securities | Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	17.5	33.2
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.7)	(1.0)
Gross unrealized losses on securities, 12 months or more, fair value	12.3	0.0
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.1)	0.0
Gross unrealized losses on securities, fair value, total	29.8	33.2
Gross unrealized losses on securities, unrealized losses, total	(0.8)	(1.0)
Fixed maturity securities | Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	282.7	347.5
Gross unrealized losses on securities, less than 12 months, unrealized losses	(5.0)	(7.9)
Gross unrealized losses on securities, 12 months or more, fair value	31.6	2.2
Gross unrealized losses on securities, 12 months or more, unrealized losses	(1.0)	(0.1)
Gross unrealized losses on securities, fair value, total	314.3	349.7
Gross unrealized losses on securities, unrealized losses, total	(6.0)	(8.0)
Fixed maturity securities | U.S. corporate
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	445.7	940.4
Gross unrealized losses on securities, less than 12 months, unrealized losses	(4.7)	(13.1)
Gross unrealized losses on securities, 12 months or more, fair value	26.9	34.1
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.6)	(2.5)
Gross unrealized losses on securities, fair value, total	472.6	974.5
Gross unrealized losses on securities, unrealized losses, total	(5.3)	(15.6)
Fixed maturity securities | Foreign corporate
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	387.8	227.3
Gross unrealized losses on securities, less than 12 months, unrealized losses	(7.8)	(4.6)
Gross unrealized losses on securities, 12 months or more, fair value	7.7	7.6
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.3)	(1.0)
Gross unrealized losses on securities, fair value, total	395.5	234.9
Gross unrealized losses on securities, unrealized losses, total	(8.1)	(5.6)
Equity securities | Non-redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on securities, less than 12 months, fair value	5.4	64.4
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.3)	(1.0)
Gross unrealized losses on securities, 12 months or more, fair value	0.0	1.9
Gross unrealized losses on securities, 12 months or more, unrealized losses	0.0	(0.1)
Gross unrealized losses on securities, fair value, total	5.4	66.3
Gross unrealized losses on securities, unrealized losses, total	$ (0.3)	$ (1.1)