Investments (Category and Duration Of Gross Unrealized Losses On Fixed Maturity Securities And Equity Securities) (Details) - USD ($) $ in Millions	Dec. 31, 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	$ 1,203.0	$ 1,755.1
Gross unrealized losses on securities, less than 12 months, unrealized losses	(11.4)	(29.2)
Gross unrealized losses on securities, 12 months or more, fair value	270.8	44.9
Gross unrealized losses on securities, 12 months or more, unrealized losses	(7.3)	(3.7)
Gross unrealized losses on securities, fair Value, total	1,473.8	1,800.0
Gross unrealized losses on securities, unrealized losses, total	(18.7)	(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	104.2	91.0
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.7)	(1.3)
Gross unrealized losses on securities, 12 months or more, fair value	43.3	0.0
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.5)	0.0
Gross unrealized losses on securities, fair Value, total	147.5	91.0
Gross unrealized losses on securities, unrealized losses, total	(1.2)	(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	24.4	98.8
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.2)	(0.9)
Gross unrealized losses on securities, 12 months or more, fair value	0.8	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	25.2	98.8
Gross unrealized losses on securities, unrealized losses, total	(0.3)	(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	27.6	0.0
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.1)	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	27.6	1.0
Gross unrealized losses on securities, unrealized losses, total	(0.1)	(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	0.0	33.2
Gross unrealized losses on securities, less than 12 months, unrealized losses	0.0	(1.0)
Gross unrealized losses on securities, 12 months or more, fair value	12.4	0.0
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.7)	0.0
Gross unrealized losses on securities, fair Value, total	12.4	33.2
Gross unrealized losses on securities, unrealized losses, total	(0.7)	(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	217.3	347.5
Gross unrealized losses on securities, less than 12 months, unrealized losses	(2.4)	(7.9)
Gross unrealized losses on securities, 12 months or more, fair value	162.9	2.2
Gross unrealized losses on securities, 12 months or more, unrealized losses	(4.9)	(0.1)
Gross unrealized losses on securities, fair Value, total	380.2	349.7
Gross unrealized losses on securities, unrealized losses, total	(7.3)	(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	562.8	940.4
Gross unrealized losses on securities, less than 12 months, unrealized losses	(4.5)	(13.1)
Gross unrealized losses on securities, 12 months or more, fair value	30.0	34.1
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.5)	(2.5)
Gross unrealized losses on securities, fair Value, total	592.8	974.5
Gross unrealized losses on securities, unrealized losses, total	(5.0)	(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	266.7	227.3
Gross unrealized losses on securities, less than 12 months, unrealized losses	(3.5)	(4.6)
Gross unrealized losses on securities, 12 months or more, fair value	19.0	7.6
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.5)	(1.0)
Gross unrealized losses on securities, fair Value, total	285.7	234.9
Gross unrealized losses on securities, unrealized losses, total	(4.0)	(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	13.8	64.4
Gross unrealized losses on securities, less than 12 months, unrealized losses	(0.2)	(1.0)
Gross unrealized losses on securities, 12 months or more, fair value	8.7	1.9
Gross unrealized losses on securities, 12 months or more, unrealized losses	(0.3)	(0.1)
Gross unrealized losses on securities, fair Value, total	22.5	66.3
Gross unrealized losses on securities, unrealized losses, total	$ (0.5)	$ (1.1)