Future Policy Benefits and Claims - Claims Development Tables (Details) $ in Millions	Dec. 31, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Claims Development
Total outstanding liabilities for unpaid claims net of reinsurance	$ 1,401.5
Long-Term Disability/Group Life Waiver
Claims Development
Net incurred claims	2,300.7
Net cumulative paid claims	1,314.6
Liability for unpaid claims net, not separately presented	251.8
Total outstanding liabilities for unpaid claims net of reinsurance	1,237.9
Long-Term Disability/Group Life Waiver | Incurral year 2014
Claims Development
Net incurred claims	200.7	$ 199.8	$ 199.3	$ 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	7,610
Net cumulative paid claims	$ 158.5	153.3	147.2	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	208.3	210.5	211.8	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	7,184
Net cumulative paid claims	$ 160.3	154.0	146.5	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	219.0	221.9	218.7	214.4	219.5	219.4	228.4	229.8
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	6,171
Net cumulative paid claims	$ 165.3	157.1	147.2	136.8	124.9	105.6	70.6	$ 16.2
Long-Term Disability/Group Life Waiver | Incurral year 2017
Claims Development
Net incurred claims	248.9	246.5	245.2	245.8	243.1	239.7	238.4
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	6,091
Net cumulative paid claims	$ 176.8	165.4	151.7	135.9	115.0	76.5	$ 17.8
Long-Term Disability/Group Life Waiver | Incurral year 2018
Claims Development
Net incurred claims	238.0	235.3	239.8	239.2	245.1	239.4
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	5,782
Net cumulative paid claims	$ 163.3	150.3	135.7	115.7	79.9	$ 20.1
Long-Term Disability/Group Life Waiver | Incurral year 2019
Claims Development
Net incurred claims	238.6	240.2	240.4	248.4	255.2
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	5,961
Net cumulative paid claims	$ 150.6	136.4	117.5	79.7	$ 19.2
Long-Term Disability/Group Life Waiver | Incurral year 2020
Claims Development
Net incurred claims	217.7	221.1	231.0	252.1
Incurred but not reported claims	$ 5.8
Cumulative number of reported claims | claim	5,939
Net cumulative paid claims	$ 130.0	113.1	78.8	$ 20.6
Long-Term Disability/Group Life Waiver | Incurral year 2021
Claims Development
Net incurred claims	221.6	244.5	259.7
Incurred but not reported claims	$ 8.5
Cumulative number of reported claims | claim	5,571
Net cumulative paid claims	$ 113.2	79.0	$ 19.8
Long-Term Disability/Group Life Waiver | Incurral year 2022
Claims Development
Net incurred claims	240.5	274.3
Incurred but not reported claims	$ 4.1
Cumulative number of reported claims | claim	5,572
Net cumulative paid claims	$ 76.6	19.6
Long-Term Disability/Group Life Waiver | Incurral year 2023
Claims Development
Net incurred claims	267.4
Incurred but not reported claims	$ 111.8
Cumulative number of reported claims | claim	3,353
Net cumulative paid claims	$ 20.0
Dental, Vision, STD, Critical Illness, Accident, Hospital Indemnity and PFML
Claims Development
Net incurred claims	1,942.9
Net cumulative paid claims	1,863.8
Total outstanding liabilities for unpaid claims net of reinsurance	79.1
Dental, Vision, STD, Critical Illness, Accident, Hospital Indemnity and PFML | Incurral year 2022
Claims Development
Net incurred claims	$ 910.6	924.4
Cumulative number of reported claims | claim	4,317,802
Net cumulative paid claims	$ 909.8	845.5
Dental, Vision, STD, Critical Illness, Accident, Hospital Indemnity and PFML | Incurral year 2023
Claims Development
Net incurred claims	1,032.3
Incurred but not reported claims	$ 57.1
Cumulative number of reported claims | claim	4,566,920
Net cumulative paid claims	$ 954.0
Group Life
Claims Development
Net incurred claims	569.0
Net cumulative paid claims	492.6
Liability for unpaid claims net, not separately presented	8.1
Total outstanding liabilities for unpaid claims net of reinsurance	84.5
Group Life | Incurral year 2022
Claims Development
Net incurred claims	284.2	279.3
Incurred but not reported claims	$ 1.1
Cumulative number of reported claims | claim	6,168
Net cumulative paid claims	$ 277.5	$ 218.3
Group Life | Incurral year 2023
Claims Development
Net incurred claims	284.8
Incurred but not reported claims	$ 33.3
Cumulative number of reported claims | claim	5,064
Net cumulative paid claims	$ 215.1