Segments (Premiums, Administrative Fees And Net Investment Income By Major Product Line Or Category) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Premiums	$ 538.5	$ 507.3	$ 1,613.2	$ 1,536.6
Administrative fees	32.5	32.8	98.3	97.0
Net investment income	153.7	152.6	466.3	461.3
Insurance Services [Member]
Premiums	537.7	504.2	1,605.9	1,527.9
Administrative fees	4.3	3.8	12.5	12.3
Net investment income	77.9	78.0	234.9	236.0
Employee Benefits [Member]
Premiums	486.2	453.9	1,453.1	1,380.6
Administrative fees	4.3	3.8	12.4	12.2
Net investment income	64.4	64.9	193.5	196.2
Employee Benefits [Member] | Group Insurance [Member]
Premiums	486.2	453.9	1,453.1	1,380.6
Employee Benefits [Member] | Group Life And ADD [Member]
Premiums	210.9	202.0	629.5	605.1
Employee Benefits [Member] | Group Long Term Disability [Member]
Premiums	194.3	187.1	584.8	565.7
Employee Benefits [Member] | Group Short Term Disability [Member]
Premiums	64.5	55.2	186.1	167.0
Employee Benefits [Member] | Group Other [Member]
Premiums	21.7	19.9	63.1	58.5
Employee Benefits [Member] | Experience Rated Refunds [Member]
Premiums	(5.2)	(10.3)	(10.4)	(15.7)
Individual Disability [Member]
Premiums	51.5	50.3	152.8	147.3
Administrative fees	0.0	0.0	0.1	0.1
Net investment income	13.5	13.1	41.4	39.8
Individual Disability [Member] | Individual Disability Insurance Policies [Member]
Premiums	51.5	50.3	152.8	147.3
Asset Management Segment [Member]
Premiums	0.8	3.1	7.3	8.7
Administrative fees	33.3	33.9	100.8	99.2
Net investment income	69.8	69.4	216.0	211.1
Asset Management Segment [Member] | Retirement Plans [Member]
Premiums	0.2	0.5	1.7	1.7
Administrative fees	25.7	25.5	76.5	74.2
Net investment income	32.2	28.1	93.2	83.9
Asset Management Segment [Member] | Individual Annuities [Member]
Premiums	0.6	2.6	5.6	7.0
Net investment income	31.0	37.2	104.2	116.9
Asset Management Segment [Member] | Other Financial Services Businesses [Member]
Administrative fees	7.6	8.4	24.3	25.0
Net investment income	6.6	4.1	18.6	10.3
Other Category [Member]
Administrative fees	(5.1)	(4.9)	(15.0)	(14.5)
Net investment income	$ 6.0	$ 5.2	$ 15.4	$ 14.2