Investments - Realized Losses Related to Other-Than-Temporary Impairments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Other-than-temporary impairments	$ (3.5)	$ (6.8)
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Other-than-temporary impairments	0.0	(2.3)
Large-cap securities | Equity securities:
Debt Securities, Available-for-sale [Line Items]
Other-than-temporary impairments	(1.5)	(0.6)
Small-cap securities | Equity securities:
Debt Securities, Available-for-sale [Line Items]
Other-than-temporary impairments	$ (2.0)	$ (3.9)