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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 489.1	$ 411.5	$ 481.4	$ 315.8
Total gains (losses) recognized in net income	0.4	(0.7)	(0.6)	0.6
Total gains (losses) recognized in accumulated other comprehensive loss	0.6	1.5	3.9	(2.0)
Purchases	68.0	62.0	149.8	164.2
Sales	(12.0)	(8.9)	(46.5)	(12.3)
Settlements	(16.8)	(10.3)	(37.0)	(21.2)
Transfers into Level III	6.3	9.8	15.8	21.7
Transfers out of Level III	(15.2)	(57.2)	(46.4)	(59.1)
Ending balance	520.4	407.7	520.4	407.7
Change in unrealized losses included in net income related to assets still held	(0.9)	(0.1)	(2.6)	(1.8)
Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	230.0	232.2	238.8	186.2
Total gains (losses) recognized in net income	0.9	(0.7)	(0.4)	(1.6)
Total gains (losses) recognized in accumulated other comprehensive loss	0.1	0.2	3.7	(1.3)
Purchases	25.4	33.9	60.2	91.9
Sales	(7.3)	(0.8)	(39.9)	(1.8)
Settlements	(16.0)	(10.3)	(35.6)	(21.2)
Transfers into Level III	5.1	0.6	13.4	2.9
Transfers out of Level III	0.0	(47.0)	(2.0)	(47.0)
Ending balance	238.2	208.1	238.2	208.1
Change in unrealized losses included in net income related to assets still held	(0.9)	(0.1)	(2.6)	(1.8)
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	7.2	0.0	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	0.0	0.0	1.5	0.0
Sales	(3.9)	0.0	(5.4)	0.0
Settlements	(0.1)	0.0	(0.3)	0.0
Transfers into Level III	1.2	1.6	1.2	1.6
Transfers out of Level III	(1.5)	0.0	(6.1)	0.0
Ending balance	2.9	1.6	2.9	1.6
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 Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	28.6	34.8	42.8	25.6
Total gains (losses) recognized in net income	0.0	0.0	0.0	0.0
Total gains (losses) recognized in accumulated other comprehensive loss	0.1	0.1	0.2	(0.3)
Purchases	21.3	0.0	30.8	0.0
Sales	(0.8)	0.0	(0.8)	0.0
Settlements	(0.7)	0.0	(1.1)	0.0
Transfers into Level III	0.0	7.6	1.2	17.2
Transfers out of Level III	(13.7)	(10.2)	(38.3)	(10.2)
Ending balance	34.8	32.3	34.8	32.3
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	223.3	144.5	187.8	102.1
Total gains (losses) recognized in net income	(0.5)	0.0	(0.2)	2.2
Total gains (losses) recognized in accumulated other comprehensive loss	0.4	1.2	0.0	(0.4)
Purchases	21.3	28.1	57.3	72.3
Sales	0.0	(8.1)	(0.4)	(10.5)
Settlements	0.0	0.0	0.0	0.0
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	244.5	165.7	244.5	165.7
Change in unrealized losses included in net income related to assets still held	$ 0.0	$ 0.0	$ 0.0	$ 0.0