Insurance Liabilities - Claims Development Tables (Details) $ in Millions	Dec. 31, 2017 USD ($) claim	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Total outstanding liabilities for unpaid claims net of reinsurance	$ 1,260.5
Long-Term Disability/Group Life Waiver
Claims Development [Line Items]
Net incurred claims	1,986.9
Net cumulative paid claims	1,039.5
Liability for unpaid claims net, not separately presented	212.9
Total outstanding liabilities for unpaid claims net of reinsurance	1,160.3
Long-Term Disability/Group Life Waiver | Incurral year 2008
Claims Development [Line Items]
Net incurred claims	199.1	$ 199.8	$ 201.6	$ 205.5	$ 209.5	$ 218.5	$ 225.9	$ 226.3	$ 222.3	$ 227.7
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	7,730
Net cumulative paid claims	$ 152.4	146.4	139.4	131.7	123.2	113.3	99.4	84.0	58.1	$ 15.1
Long-Term Disability/Group Life Waiver | Incurral year 2009
Claims Development [Line Items]
Net incurred claims	205.6	205.8	208.5	214.1	217.7	224.8	224.2	224.4	218.6
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	6,554
Net cumulative paid claims	$ 149.8	141.8	133.1	124.6	113.8	101.0	82.6	55.2	$ 13.4
Long-Term Disability/Group Life Waiver | Incurral year 2010
Claims Development [Line Items]
Net incurred claims	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	5,644
Net cumulative paid claims	$ 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 [Line Items]
Net incurred claims	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	6,286
Net cumulative paid claims	$ 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 [Line Items]
Net incurred claims	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	6,441
Net cumulative paid claims	$ 112.9	104.6	93.7	80.8	55.1	$ 13.8
Long-Term Disability/Group Life Waiver | Incurral year 2013
Claims Development [Line Items]
Net incurred claims	182.3	190.7	188.4	203.3	219.3
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	7,044
Net cumulative paid claims	$ 106.4	97.0	81.4	55.0	$ 12.5
Long-Term Disability/Group Life Waiver | Incurral year 2014
Claims Development [Line Items]
Net incurred claims	218.1	214.4	231.4	242.2
Incurred but not reported claims	$ 1.7
Cumulative number of reported claims | claim	7,591
Net cumulative paid claims	$ 111.8	96.3	66.0	$ 16.1
Long-Term Disability/Group Life Waiver | Incurral year 2015
Claims Development [Line Items]
Net incurred claims	217.2	227.2	231.0
Incurred but not reported claims	$ 3.7
Cumulative number of reported claims | claim	7,146
Net cumulative paid claims	$ 98.0	67.0	$ 16.9
Long-Term Disability/Group Life Waiver | Incurral year 2016
Claims Development [Line Items]
Net incurred claims	228.4	229.8
Incurred but not reported claims	$ 6.2
Cumulative number of reported claims | claim	6,076
Net cumulative paid claims	$ 70.6	16.2
Long-Term Disability/Group Life Waiver | Incurral year 2017
Claims Development [Line Items]
Net incurred claims	238.4
Incurred but not reported claims	$ 81.2
Cumulative number of reported claims | claim	3,640
Net cumulative paid claims	$ 17.8
Dental/Vision/Short-Term Disability/Critical Illness
Claims Development [Line Items]
Net incurred claims	1,136.3
Net cumulative paid claims	1,082.7
Total outstanding liabilities for unpaid claims net of reinsurance	53.6
Dental/Vision/Short-Term Disability/Critical Illness | Incurral year 2016
Claims Development [Line Items]
Net incurred claims	540.5	544.2
Incurred but not reported claims	$ 0.1
Cumulative number of reported claims | claim	2,563,426
Net cumulative paid claims	$ 540.4	495.4
Dental/Vision/Short-Term Disability/Critical Illness | Incurral year 2017
Claims Development [Line Items]
Net incurred claims	595.8
Incurred but not reported claims	$ 32.1
Cumulative number of reported claims | claim	2,648,418
Net cumulative paid claims	$ 542.3
Group Life
Claims Development [Line Items]
Net incurred claims	460.2
Net cumulative paid claims	414.5
Liability for unpaid claims net, not separately presented	0.9
Total outstanding liabilities for unpaid claims net of reinsurance	46.6
Group Life | Incurral year 2016
Claims Development [Line Items]
Net incurred claims	220.4	222.6
Incurred but not reported claims	$ 0.4
Cumulative number of reported claims | claim	5,155
Net cumulative paid claims	$ 219.3	$ 179.0
Group Life | Incurral year 2017
Claims Development [Line Items]
Net incurred claims	239.8
Incurred but not reported claims	$ 21.1
Cumulative number of reported claims | claim	5,123
Net cumulative paid claims	$ 195.2