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	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 481.4	$ 315.8
Total gains (losses) recognized in net income	(1.0)	1.3
Total gains (losses) recognized in accumulated other comprehensive loss	3.3	(3.5)
Purchases	81.8	102.2
Sales	(34.5)	(3.4)
Settlements	(20.2)	(10.9)
Transfers into Level III	9.5	11.9
Transfers out of Level III	(31.2)	(1.9)
Ending balance	489.1	411.5
Change in unrealized losses included in net income related to assets still held	(1.7)	(1.7)
Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	238.8	186.2
Total gains (losses) recognized in net income	(1.3)	(0.9)
Total gains (losses) recognized in accumulated other comprehensive loss	3.6	(1.5)
Purchases	34.8	58.0
Sales	(32.6)	(1.0)
Settlements	(19.6)	(10.9)
Transfers into Level III	8.3	2.3
Transfers out of Level III	(2.0)	0.0
Ending balance	230.0	232.2
Change in unrealized losses included in net income related to assets still held	(1.7)	(1.7)
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	12.0	0.0
Total gains (losses) recognized in net income	0.0	0.0
Total gains (losses) recognized in accumulated other comprehensive loss	0.0	0.0
Purchases	1.5	0.0
Sales	(1.5)	0.0
Settlements	(0.2)	0.0
Transfers into Level III	0.0	0.0
Transfers out of Level III	(4.6)	0.0
Ending balance	7.2	0.0
Change in unrealized losses included in net income related to assets still held	0.0	0.0
Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0	1.9
Total gains (losses) recognized in net income	0.0	0.0
Total gains (losses) recognized in accumulated other comprehensive loss	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Transfers into Level III	0.0	0.0
Transfers out of Level III	0.0	(1.9)
Ending balance	0.0	0.0
Change in unrealized losses included in net income related to assets still held	0.0	0.0
Other Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	42.8	25.6
Total gains (losses) recognized in net income	0.0	0.0
Total gains (losses) recognized in accumulated other comprehensive loss	0.1	(0.4)
Purchases	9.5	0.0
Sales	0.0	0.0
Settlements	(0.4)	0.0
Transfers into Level III	1.2	9.6
Transfers out of Level III	(24.6)	0.0
Ending balance	28.6	34.8
Change in unrealized losses included in net income related to assets still held	0.0	0.0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	187.8	102.1
Total gains (losses) recognized in net income	0.3	2.2
Total gains (losses) recognized in accumulated other comprehensive loss	(0.4)	(1.6)
Purchases	36.0	44.2
Sales	(0.4)	(2.4)
Settlements	0.0	0.0
Transfers into Level III	0.0	0.0
Transfers out of Level III	0.0	0.0
Ending balance	223.3	144.5
Change in unrealized losses included in net income related to assets still held	$ 0.0	$ 0.0