Investments - OTTI OCI (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Other than Temporary Impairment, Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning	$ 19.8	$ 19.5	$ 20.0	$ 19.4
Additional Credit Impairments [Abstract]
On securities not previously impaired	0.6	1.1	0.8	1.2
On securities previously impaired	0.6	0	1.0	0.6
Reductions [Abstract]
Securities sold, matured, prepaid, or paid down	(0.8)	(0.6)	(1.6)	(1.2)
Balance, ending	$ 20.2	$ 20.0	$ 20.2	$ 20.0