Insurance Liabilities - Claims Development Tables (Details) $ in Millions	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Claims Development
Total outstanding liabilities for unpaid claims net of reinsurance	$ 1,333.2
Long-Term Disability/Group Life Waiver
Claims Development
Net incurred claims	2,125.4
Net cumulative paid claims	1,156.8
Liability for unpaid claims net, not separately presented	240.2
Total outstanding liabilities for unpaid claims net of reinsurance	1,208.8
Long-Term Disability/Group Life Waiver | Incurral year 2011
Claims Development
Net incurred claims	167.0	$ 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	6,294
Net cumulative paid claims	$ 129.7	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	173.1	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	6,445
Net cumulative paid claims	$ 131.5	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	173.4	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	$ 129.0	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	202.0	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	7,603
Net cumulative paid claims	$ 140.8	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	210.0	208.2	215.3	217.2	227.2	231.0
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	7,179
Net cumulative paid claims	$ 137.1	126.8	114.6	98.0	67.0	$ 16.9
Long-Term Disability/Group Life Waiver | Incurral year 2016
Claims Development
Net incurred claims	214.4	219.5	219.4	228.4	229.8
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	6,163
Net cumulative paid claims	$ 136.8	124.9	105.6	70.6	$ 16.2
Long-Term Disability/Group Life Waiver | Incurral year 2017
Claims Development
Net incurred claims	245.8	243.1	239.7	238.4
Incurred but not reported claims	$ 4.4
Cumulative number of reported claims | claim	6,074
Net cumulative paid claims	$ 135.9	115.0	76.5	$ 17.8
Long-Term Disability/Group Life Waiver | Incurral year 2018
Claims Development
Net incurred claims	239.2	245.1	239.4
Incurred but not reported claims	$ 6.6
Cumulative number of reported claims | claim	5,738
Net cumulative paid claims	$ 115.7	79.9	$ 20.1
Long-Term Disability/Group Life Waiver | Incurral year 2019
Claims Development
Net incurred claims	248.4	255.2
Incurred but not reported claims	$ 3.2
Cumulative number of reported claims | claim	5,858
Net cumulative paid claims	$ 79.7	19.2
Long-Term Disability/Group Life Waiver | Incurral year 2020
Claims Development
Net incurred claims	252.1
Incurred but not reported claims	$ 87.0
Cumulative number of reported claims | claim	3,588
Net cumulative paid claims	$ 20.6
Dental/Vision/Short-Term Disability/Critical Illness/Accident
Claims Development
Net incurred claims	1,391.4
Net cumulative paid claims	1,321.0
Total outstanding liabilities for unpaid claims net of reinsurance	70.4
Dental/Vision/Short-Term Disability/Critical Illness/Accident | Incurral year 2019
Claims Development
Net incurred claims	$ 711.6	724.7
Cumulative number of reported claims | claim	3,363,947
Net cumulative paid claims	$ 711.5	653.5
Dental/Vision/Short-Term Disability/Critical Illness/Accident | Incurral year 2020
Claims Development
Net incurred claims	679.8
Incurred but not reported claims	$ 45.0
Cumulative number of reported claims | claim	2,899,957
Net cumulative paid claims	$ 609.5
Group Life
Claims Development
Net incurred claims	495.2
Net cumulative paid claims	442.3
Liability for unpaid claims net, not separately presented	1.1
Total outstanding liabilities for unpaid claims net of reinsurance	54.0
Group Life | Incurral year 2019
Claims Development
Net incurred claims	224.6	228.3
Incurred but not reported claims	$ 0.7
Cumulative number of reported claims | claim	5,117
Net cumulative paid claims	$ 223.0	$ 181.7
Group Life | Incurral year 2020
Claims Development
Net incurred claims	270.6
Incurred but not reported claims	$ 21.9
Cumulative number of reported claims | claim	5,505
Net cumulative paid claims	$ 219.3