Investing Activities - Net Realized Investment Gains/Losses (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
Investments, Debt and Equity Securities [Abstract]
Total other-than-temporary debt impairments	$ 0	$ (3.0)	$ (0.3)	$ (3.7)
Portion of loss recognized in OCI	(0.2)	2.4	(0.9)	2.1
Net debt impairments recognized in earnings	(0.2)	(0.6)	(1.2)	(1.6)
Debt security impairments:
U.S. government and agency	0	0	0	0
State and political subdivision	0	0	0	0
Foreign government	0	0	0	0
Corporate	0	0	0	0
CMBS	(0.1)	0	(0.2)	(0.1)
RMBS	(0.1)	(0.5)	(0.8)	(1.3)
CDO/CLO	0	0	(0.2)	0
Other asset-backed	0	(0.1)	0	(0.2)
Net debt security impairments	(0.2)	(0.6)	(1.2)	(1.6)
Equity security impairments	0	0	0	0
Net OTTI losses recognized in earnings	(0.2)	(0.6)	(1.2)	(1.6)
Debt security transaction gains	0.4	0.3	5.6	0.6
Debt security transaction losses	(0.2)	0	(0.3)	(0.1)
Limited partnerships and other investment gains	0	0	0	0
Limited partnerships and other investment losses	0	(0.1)	0	(0.1)
Net transaction gains	0.2	0.2	5.3	0.4
Derivative instruments	4.8	3.9	(16.1)	(15.5)
Embedded derivatives	5.9	(23.5)	5.3	(0.5)
Related party reinsurance derivatives	0	6.0	0	0.7
Net realized investment gains (losses), excluding impairment losses	10.9	(13.4)	(5.5)	(14.9)
Net realized investment gains (losses), including impairment losses	$ 10.7	$ (14.0)	$ (6.7)	$ (16.5)