Investments - OTTI OCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Other than Temporary Impairment, Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning	$ 21.4	$ 27.4	$ 22.4	$ 28.0
Additional Credit Impairments [Abstract]
On securities not previously impaired	0.0	0.0	0.0	0.7
On securities previously impaired	0.2	0.3	1.0	0.5
Reductions [Abstract]
Increase in cash flows	(0.1)	0.0	(0.2)	0.0
Securities sold, matured, prepaid, or paid down	(0.8)	(1.1)	(2.5)	(2.6)
Balance, ending	$ 20.7	$ 26.6	$ 20.7	$ 26.6