Investments - OTTI OCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance, beginning	$ 19.3	$ 22.4	$ 28.0
Additional Credit Impairments [Abstract]
On securities not previously impaired	0.0	0.0	0.7
On securities previously impaired	0.8	1.3	0.9
Reductions [Abstract]
Increase in cash flows	1.6	0.2	0.6
Securities sold, matured, prepaid, or paid down	9.3	4.2	6.6
Balance, ending	$ 9.2	$ 19.3	$ 22.4