Investments - OTTI OCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Other than Temporary Impairment, Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning	$ 18.7	$ 21.4	$ 19.3	$ 22.4
Additional Credit Impairments [Abstract]
On securities previously impaired	0.0	0.2	0.5	1.0
Reductions [Abstract]
Increase in cash flows	1.3	0.1	1.4	0.2
Securities sold, matured, prepaid, or paid down	4.3	0.8	5.3	2.5
Balance, ending	$ 13.1	$ 20.7	$ 13.1	$ 20.7