Investments (Categories Of Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	$ 524.1	$ 533.4	$ 649.5
Investment expenses	(21.9)	(17.7)	(23.3)
Net investment income	493.8	515.7	626.2
Fixed maturity securities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	411.8	419.3	486.2
Equity securities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	22.8	25.2	29.9
Commercial mortgage loans on real estate
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	31.5	41.7	72.7
Short-term investments
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	7.2	5.5	2.0
Other investments
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	25.2	24.2	40.3
Cash and cash equivalents
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	15.8	17.5	18.4
Revenues (expenses) from consolidated investment entities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Revenues from consolidated investment entities	9.8	0.0	0.0
Expenses from consolidated investment entities	(8.4)	$ 0.0	$ 0.0
Net revenues and expenses	1.4
CLO
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	0.6
Real estate fund
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total Investment Income	0.5
Investment management fees
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Expenses from consolidated investment entities	$ (0.3)