Segments (Premiums, Administrative Fees And Net Investment Income By Major Product Line Or Category) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Premiums	$ 528.7	$ 539.8	$ 1,630.1	$ 1,610.6
Administrative fees	27.8	28.5	86.2	87.4
Net investment income	159.8	147.3	470.9	456.9
Insurance Services [Member]
Premiums	527.5	538.3	1,625.1	1,604.4
Administrative fees	3.1	3.0	10.4	8.6
Net investment income	86.0	85.8	254.2	255.6
Individual Policies [Member]
Premiums	44.4	44.2	130.4	129.0
Administrative fees	0.1	0.1	0.2	0.2
Net investment income	13.8	13.1	40.2	39.6
Group Insurance [Member]
Premiums	483.1	494.1	1,494.7	1,475.4
Administrative fees	3.0	2.9	10.2	8.4
Net investment income	72.2	72.7	214.0	216.0
Group Life And ADD [Member]
Premiums	218.3	224.7	667.0	670.8
Group Long Term Disability [Member]
Premiums	199.1	200.5	603.5	602.0
Group Short Term Disability [Member]
Premiums	52.5	52.4	160.3	156.3
Group Other [Member]
Premiums	19.7	20.5	59.2	60.5
Experience Rated Refunds [Member]
Premiums	(6.5)	(4.0)	4.7	(14.2)
Asset Management [Member]
Premiums	1.2	1.5	5.0	6.2
Administrative fees	29.2	29.7	89.1	91.2
Net investment income	72.3	59.0	210.0	193.5
Retirement Plans [Member]
Premiums	0.1	0.7	1.3	1.4
Administrative fees	21.5	22.3	66.0	68.5
Net investment income	24.2	22.5	71.0	66.9
Individual Annuities [Member]
Premiums	1.1	0.8	3.7	4.8
Net investment income	44.4	33.4	129.0	117.2
Other Financial Services Businesses [Member]
Administrative fees	7.7	7.4	23.1	22.7
Net investment income	3.7	3.1	10.0	9.4
Other Category [Member]
Administrative fees	(4.5)	(4.2)	(13.3)	(12.4)
Net investment income	$ 1.5	$ 2.5	$ 6.7	$ 7.8