Investing Activities - Net Realized Investment Gains/Losses (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Total other-than-temporary debt impairments	$ (0.3)	$ (0.7)
Portion of loss recognized in OCI	(0.7)	(0.3)
Net debt impairments recognized in earnings	(1.0)	(1.0)
Debt security impairments:
U.S. government and agency	0	0
State and political subdivision	0	0
Foreign government	0	0
Corporate	0	0
CMBS	(0.1)	(0.1)
RMBS	(0.7)	(0.8)
CDO/CLO	(0.2)	0
Other asset-backed	0	(0.1)
Net debt security impairments	(1.0)	(1.0)
Equity security impairments	0	0
Net OTTI losses recognized in earnings	(1.0)	(1.0)
Debt security transaction gains	5.2	0.3
Debt security transaction losses	(0.1)	(0.1)
Limited partnerships and other investment gains	0	0
Limited partnerships and other investment losses	0	0
Net transaction gains (losses)	5.1	0.2
Derivative instruments	(20.9)	(19.4)
Embedded derivatives	(0.6)	23.0
Related party reinsurance derivatives	0	(5.3)
Net realized investment gains (losses), excluding impairment losses	(16.4)	(1.5)
Net realized investment gains (losses), including impairment losses	$ (17.4)	$ (2.5)