Fair Value (Reconciliation Of The Beginning And Ending Balances Of Assets Measured At Fair Value On A Recurring Basis Using Level III Inputs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 202.8	$ 177.9	$ 155.5
Gains (losses) recognized in net income	0.1	(5.1)	(35.2)
Gains (losses) recognized in accumulated other comprehensive income	4.4	11.7	6.6
Purchases	214.1	94.9	71.1
Sales	(27.5)	(62.2)	(11.9)
Settlements	(53.2)	(15.1)	(24.2)
Transfers into Level III	17.9	24.8	38.5
Transfers out of Level III	(42.8)	(24.1)	(22.5)
Ending balance	315.8	202.8	177.9
Change in unrealized losses included in net income related to assets still held	(2.0)	(11.8)	(37.4)
Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	144.6	115.2	121.1
Gains (losses) recognized in net income	1.4	(4.4)	(30.3)
Gains (losses) recognized in accumulated other comprehensive income	0.7	8.5	(3.5)
Purchases	132.6	68.9	51.9
Sales	(11.7)	(48.0)	(4.8)
Settlements	(51.6)	(11.0)	(15.5)
Transfers into Level III	4.8	24.8	3.0
Transfers out of Level III	(34.6)	(9.4)	(6.7)
Ending balance	186.2	144.6	115.2
Change in unrealized losses included in net income related to assets still held	(0.6)	(11.1)	(30.8)
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0	0.0	4.3
Gains (losses) recognized in net income	0.0	0.0	0.0
Gains (losses) recognized in accumulated other comprehensive income	0.0	0.0	0.0
Purchases	0.0	0.0	0.0
Sales	0.0	0.0	0.0
Settlements	0.0	0.0	(1.9)
Transfers into Level III	0.0	0.0	13.1
Transfers out of Level III	0.0	0.0	(15.5)
Ending balance	0.0	0.0	0.0
Change in unrealized losses included in net income related to assets still held	0.0	0.0	0.0
Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	3.3	6.5	0.0
Gains (losses) recognized in net income	0.0	0.0	0.0
Gains (losses) recognized in accumulated other comprehensive income	0.0	0.0	0.0
Purchases	1.1	3.6	0.0
Sales	(1.1)	0.0	0.0
Settlements	(1.4)	(3.7)	(6.1)
Transfers into Level III	0.0	0.0	12.6
Transfers out of Level III	0.0	(3.1)	0.0
Ending balance	1.9	3.3	6.5
Change in unrealized losses included in net income related to assets still held	0.0	0.0	0.0
Other Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	6.6	14.8	3.9
Gains (losses) recognized in net income	0.2	0.0	(0.1)
Gains (losses) recognized in accumulated other comprehensive income	(0.2)	0.4	0.6
Purchases	28.3	6.5	1.6
Sales	(0.9)	(3.6)	0.0
Settlements	(0.2)	(0.4)	(0.7)
Transfers into Level III	0.0	0.0	9.8
Transfers out of Level III	(8.2)	(11.1)	(0.3)
Ending balance	25.6	6.6	14.8
Change in unrealized losses included in net income related to assets still held	0.0	0.0	(0.1)
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	48.3	41.4	26.2
Gains (losses) recognized in net income	(1.5)	(0.7)	(4.8)
Gains (losses) recognized in accumulated other comprehensive income	3.9	2.8	9.5
Purchases	52.1	15.9	17.6
Sales	(13.8)	(10.6)	(7.1)
Settlements	0.0	0.0	0.0
Transfers into Level III	13.1	0.0	0.0
Transfers out of Level III	0.0	(0.5)	0.0
Ending balance	102.1	48.3	41.4
Change in unrealized losses included in net income related to assets still held	$ (1.4)	$ (0.7)	$ (6.5)