Investments - OTTI OCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other than Temporary Impairment, Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning	$ 7.3	$ 18.7	$ 9.2	$ 19.3
Additional Credit Impairments [Abstract]
On securities previously impaired	0.0	0.0	0.0	0.5
Reductions [Abstract]
Increase in cash flows	0.0	1.3	0.2	1.4
Securities sold, matured, prepaid, or paid down	0.4	4.3	2.1	5.3
Balance, ending	$ 6.9	$ 13.1	$ 6.9	$ 13.1