Investments - OTTI OCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Other than Temporary Impairment, Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning	$ 7.7	$ 19.1	$ 9.2	$ 19.3
Additional Credit Impairments [Abstract]
On securities previously impaired	0.0	0.2	0.0	0.5
Reductions [Abstract]
Increase in cash flows	0.1	0.0	0.2	0.1
Securities sold, matured, prepaid, or paid down	0.3	0.6	1.7	1.0
Balance, ending	$ 7.3	$ 18.7	$ 7.3	$ 18.7