Income Taxes - Temporary Differences (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Insurance reserves	$ 224.0	$ 197.1
Investments	141.1	197.9
Postemployment benefits	71.4	69.6
Other assets	7.4	8.6
Total gross assets before valuation allowance	443.9	473.2
Valuation allowance, deferred tax assets	5.0	7.1
Assets, net of valuation allowance	438.9	466.1
Net unrealized investment (gains) losses	(346.1)	(208.0)
Deferred policy acquisition costs	(315.6)	(391.1)
Total gross liabilities	(661.7)	(599.1)
Net deferred income tax asset (liability)	$ (222.8)	$ (133.0)