Insurance Liabilities - Reconciliation of Unpaid Claims to Liability for Unpaid Claims Table (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid claims	$ 1,260.5
Reconciling items
Reinsurance recoverable on unpaid claims	69.8
Impact of discounting	(226.2)
Liability for unpaid claims - short duration contracts	1,104.1
Insurance contracts other than short duration	1,026.4
Liability for unpaid claims	2,130.5	$ 2,001.3	$ 1,872.2	$ 1,771.4
Long-Term Disability/Group Life Waiver
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid claims	1,160.3
Reconciling items
Reinsurance recoverable on unpaid claims	69.1
Impact of discounting	(226.2)	(234.1)
Liability for unpaid claims - short duration contracts	1,003.2	992.7
Dental/Vision/Short-Term Disability/Critical Illness
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid claims	53.6
Reconciling items
Liability for unpaid claims - short duration contracts	53.6	48.8
Group Life
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities for unpaid claims	46.6
Reconciling items
Reinsurance recoverable on unpaid claims	0.7
Liability for unpaid claims - short duration contracts	$ 47.3	$ 45.9