Insurance Liabilities - Discounting Table (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying amount of liabilities for unpaid claims	$ 1,104.1
Aggregate amount of discount	226.2
Long-Term Disability/Group Life Waiver
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying amount of liabilities for unpaid claims	1,003.2	$ 992.7
Aggregate amount of discount	226.2	234.1
Interest accretion	$ 35.0	$ 36.3	$ 37.6
Long-Term Disability/Group Life Waiver | Minimum
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Discount rate	3.30%	3.30%
Long-Term Disability/Group Life Waiver | Maximum
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Discount rate	7.00%	7.00%
Dental/Vision/Short-Term Disability/Critical Illness
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying amount of liabilities for unpaid claims	$ 53.6	$ 48.8
Group Life
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying amount of liabilities for unpaid claims	$ 47.3	$ 45.9