Investments - OTTI OCI (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Other than Temporary Impairment, Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning	$ 102.8	$ 114.7
Additional credit impairments:
On securities not previously impaired	1.1	0.2
On securities previously impaired	1.0	3.0
Reductions:
Securities intent impaired	0	0
Securities sold, matured, prepaid, or paid down	3.8	5.5
Balance, ending	$ 101.1	$ 112.4