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, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 315.8	$ 202.8	$ 177.9
Gains (losses) recognized in net income	(2.2)	0.1	(5.1)
Gains (losses) recognized in accumulated other comprehensive loss/income	(3.0)	4.4	11.7
Purchases	397.1	214.1	94.9
Sales	(142.1)	(27.5)	(62.2)
Settlements	(58.1)	(53.2)	(15.1)
Transfers into Level III	44.7	17.9	24.8
Transfers out of Level III	(70.8)	(42.8)	(24.1)
Ending balance	481.4	315.8	202.8
Change in unrealized losses included in net income related to assets still held	(2.0)	(2.0)	(11.8)
Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	186.2	144.6	115.2
Gains (losses) recognized in net income	(2.9)	1.4	(4.4)
Gains (losses) recognized in accumulated other comprehensive loss/income	(2.0)	0.7	8.5
Purchases	170.2	132.6	68.9
Sales	(5.4)	(11.7)	(48.0)
Settlements	(56.8)	(51.6)	(11.0)
Transfers into Level III	6.6	4.8	24.8
Transfers out of Level III	(57.1)	(34.6)	(9.4)
Ending balance	238.8	186.2	144.6
Change in unrealized losses included in net income related to assets still held	(2.0)	(0.6)	(11.1)
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0	0.0	0.0
Gains (losses) recognized in net income	0.0	0.0	0.0
Gains (losses) recognized in accumulated other comprehensive loss/income	0.0	0.0	0.0
Purchases	4.3	0.0	0.0
Sales	0.0	0.0	0.0
Settlements	0.0	0.0	0.0
Transfers into Level III	9.3	0.0	0.0
Transfers out of Level III	(1.6)	0.0	0.0
Ending balance	12.0	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	1.9	3.3	6.5
Gains (losses) recognized in net income	0.0	0.0	0.0
Gains (losses) recognized in accumulated other comprehensive loss/income	0.0	0.0	0.0
Purchases	0.0	1.1	3.6
Sales	0.0	(1.1)	0.0
Settlements	0.0	(1.4)	(3.7)
Transfers into Level III	0.0	0.0	0.0
Transfers out of Level III	(1.9)	0.0	(3.1)
Ending balance	0.0	1.9	3.3
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	25.6	6.6	14.8
Gains (losses) recognized in net income	0.0	0.2	0.0
Gains (losses) recognized in accumulated other comprehensive loss/income	(0.5)	(0.2)	0.4
Purchases	0.0	28.3	6.5
Sales	0.0	(0.9)	(3.6)
Settlements	(0.9)	(0.2)	(0.4)
Transfers into Level III	28.8	0.0	0.0
Transfers out of Level III	(10.2)	(8.2)	(11.1)
Ending balance	42.8	25.6	6.6
Change in unrealized losses included in net income related to assets still held	0.0	0.0	0.0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	102.1	48.3	41.4
Gains (losses) recognized in net income	0.7	(1.5)	(0.7)
Gains (losses) recognized in accumulated other comprehensive loss/income	(0.5)	3.9	2.8
Purchases	222.6	52.1	15.9
Sales	(136.7)	(13.8)	(10.6)
Settlements	(0.4)	0.0	0.0
Transfers into Level III	0.0	13.1	0.0
Transfers out of Level III	0.0	0.0	(0.5)
Ending balance	187.8	102.1	48.3
Change in unrealized losses included in net income related to assets still held	$ 0.0	$ (1.4)	$ (0.7)