Insurance Liabilities - Claims Development Tables (Details) $ in Millions	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)
Claims Development
Total outstanding liabilities for unpaid claims net of reinsurance	$ 1,309.2
Long-Term Disability/Group Life Waiver
Claims Development
Net incurred claims	2,040.3
Net cumulative paid claims	1,089.6
Liability for unpaid claims net, not separately presented	237.8
Total outstanding liabilities for unpaid claims net of reinsurance	1,188.5
Long-Term Disability/Group Life Waiver | Incurral year 2010
Claims Development
Net incurred claims	151.8	$ 152.1	$ 153.4	$ 154.1	$ 155.7	$ 162.7	$ 172.0	$ 176.2	$ 176.7	$ 184.1
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	5,649
Net cumulative paid claims	$ 116.7	112.1	107.2	100.9	94.2	85.9	78.4	67.1	46.5	$ 10.4
Long-Term Disability/Group Life Waiver | Incurral year 2011
Claims Development
Net incurred claims	166.5	167.6	169.6	172.3	178.4	184.8	185.4	192.6	203.7
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	6,290
Net cumulative paid claims	$ 125.4	119.3	112.6	105.2	95.4	85.7	72.5	50.0	$ 11.2
Long-Term Disability/Group Life Waiver | Incurral year 2012
Claims Development
Net incurred claims	171.9	173.3	174.8	181.8	189.5	191.1	200.0	217.9
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	6,444
Net cumulative paid claims	$ 126.1	120.0	112.9	104.6	93.7	80.8	55.1	$ 13.8
Long-Term Disability/Group Life Waiver | Incurral year 2013
Claims Development
Net incurred claims	177.1	179.5	182.3	190.7	188.4	203.3	219.3
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	7,050
Net cumulative paid claims	$ 123.2	116.4	106.4	97.0	81.4	55.0	$ 12.5
Long-Term Disability/Group Life Waiver | Incurral year 2014
Claims Development
Net incurred claims	201.9	206.2	218.1	214.4	231.4	242.2
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	7,596
Net cumulative paid claims	$ 132.4	122.3	111.8	96.3	66.0	$ 16.1
Long-Term Disability/Group Life Waiver | Incurral year 2015
Claims Development
Net incurred claims	208.2	215.3	217.2	227.2	231.0
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	7,174
Net cumulative paid claims	$ 126.8	114.6	98.0	67.0	$ 16.9
Long-Term Disability/Group Life Waiver | Incurral year 2016
Claims Development
Net incurred claims	219.5	219.4	228.4	229.8
Incurred but not reported claims	$ 5.4
Cumulative number of reported claims | claim	6,161
Net cumulative paid claims	$ 124.9	105.6	70.6	$ 16.2
Long-Term Disability/Group Life Waiver | Incurral year 2017
Claims Development
Net incurred claims	243.1	239.7	238.4
Incurred but not reported claims	$ 8.0
Cumulative number of reported claims | claim	6,053
Net cumulative paid claims	$ 115.0	76.5	$ 17.8
Long-Term Disability/Group Life Waiver | Incurral year 2018
Claims Development
Net incurred claims	245.1	239.4
Incurred but not reported claims	$ 3.8
Cumulative number of reported claims | claim	5,668
Net cumulative paid claims	$ 79.9	20.1
Long-Term Disability/Group Life Waiver | Incurral year 2019
Claims Development
Net incurred claims	255.2
Incurred but not reported claims	$ 104.6
Cumulative number of reported claims | claim	3,472
Net cumulative paid claims	$ 19.2
Dental/Vision/Short-Term Disability/Critical Illness/Accident
Claims Development
Net incurred claims	1,364.7
Net cumulative paid claims	1,293.5
Total outstanding liabilities for unpaid claims net of reinsurance	71.2
Dental/Vision/Short-Term Disability/Critical Illness/Accident | Incurral year 2018
Claims Development
Net incurred claims	$ 640.0	648.3
Cumulative number of reported claims | claim	3,032,617
Net cumulative paid claims	$ 640.0	589.1
Dental/Vision/Short-Term Disability/Critical Illness/Accident | Incurral year 2019
Claims Development
Net incurred claims	724.7
Incurred but not reported claims	$ 45.0
Cumulative number of reported claims | claim	3,184,519
Net cumulative paid claims	$ 653.5
Group Life
Claims Development
Net incurred claims	466.7
Net cumulative paid claims	418.2
Liability for unpaid claims net, not separately presented	1.0
Total outstanding liabilities for unpaid claims net of reinsurance	49.5
Group Life | Incurral year 2018
Claims Development
Net incurred claims	238.4	239.6
Incurred but not reported claims	$ 0.7
Cumulative number of reported claims | claim	5,220
Net cumulative paid claims	$ 236.5	$ 193.9
Group Life | Incurral year 2019
Claims Development
Net incurred claims	228.3
Incurred but not reported claims	$ 23.0
Cumulative number of reported claims | claim	4,625
Net cumulative paid claims	$ 181.7