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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 520.4	$ 407.7	$ 481.4	$ 315.8
Total gains (losses) recognized in net income	(0.5)	(0.5)	(1.1)	0.1
Total gains (losses) recognized in accumulated other comprehensive loss	4.8	(0.6)	8.7	(2.6)
Purchases	47.9	69.7	197.7	233.9
Sales	(3.3)	(11.4)	(49.8)	(23.7)
Settlements	(18.6)	(6.6)	(55.6)	(27.8)
Transfers into Level III	4.1	13.8	19.9	35.5
Transfers out of Level III	(27.8)	0.0	(74.2)	(59.1)
Ending balance	527.0	472.1	527.0	472.1
Change in unrealized losses included in net income related to assets still held	(0.6)	(0.1)	(3.2)	(1.9)
Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	238.2	208.1	238.8	186.2
Total gains (losses) recognized in net income	(0.4)	0.0	(0.8)	(1.6)
Total gains (losses) recognized in accumulated other comprehensive loss	(1.4)	0.2	2.3	(1.1)
Purchases	11.5	46.7	71.7	138.6
Sales	(2.8)	(3.3)	(42.7)	(5.1)
Settlements	(14.3)	(6.0)	(49.9)	(27.2)
Transfers into Level III	0.0	2.2	13.4	5.1
Transfers out of Level III	(0.5)	0.0	(2.5)	(47.0)
Ending balance	230.3	247.9	230.3	247.9
Change in unrealized losses included in net income related to assets still held	(0.6)	(0.1)	(3.2)	(1.9)
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2.9	1.6	12.0	0.0
Total gains (losses) recognized in net income	0.0	0.0	0.0	0.0
Total gains (losses) recognized in accumulated other comprehensive loss	0.0	0.0	0.0	0.0
Purchases	2.1	3.6	3.6	3.6
Sales	0.0	0.0	(5.4)	0.0
Settlements	(0.1)	0.0	(0.4)	0.0
Transfers into Level III	0.0	0.0	1.2	1.6
Transfers out of Level III	0.0	0.0	(6.1)	0.0
Ending balance	4.9	5.2	4.9	5.2
Change in unrealized losses included in net income related to assets still held	0.0	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			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	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	34.8	32.3	42.8	25.6
Total gains (losses) recognized in net income	(0.1)	0.0	(0.1)	0.0
Total gains (losses) recognized in accumulated other comprehensive loss	0.1	(0.1)	0.3	(0.4)
Purchases	4.8	0.0	35.6	0.0
Sales	(0.4)	0.0	(1.2)	0.0
Settlements	(4.2)	(0.5)	(5.3)	(0.5)
Transfers into Level III	4.1	11.6	5.3	28.8
Transfers out of Level III	(27.3)	0.0	(65.6)	(10.2)
Ending balance	11.8	43.3	11.8	43.3
Change in unrealized losses included in net income related to assets still held	0.0	0.0	0.0	0.0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	244.5	165.7	187.8	102.1
Total gains (losses) recognized in net income	0.0	(0.5)	(0.2)	1.7
Total gains (losses) recognized in accumulated other comprehensive loss	6.1	(0.7)	6.1	(1.1)
Purchases	29.5	19.4	86.8	91.7
Sales	(0.1)	(8.1)	(0.5)	(18.6)
Settlements	0.0	(0.1)	0.0	(0.1)
Transfers into Level III	0.0	0.0	0.0	0.0
Transfers out of Level III	0.0	0.0	0.0	0.0
Ending balance	280.0	175.7	280.0	175.7
Change in unrealized losses included in net income related to assets still held	$ 0.0	$ 0.0	$ 0.0	$ 0.0