Investing Activities - Net Realized Investment Gains/Losses (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Investments, Debt and Equity Securities [Abstract]
Total other-than-temporary debt impairments	$ 0		$ 0		$ 0		$ (0.3)
Portion of gains (losses) recognized in OCI	0		(0.2)		0		(0.9)
Net debt impairments recognized in earnings	0		(0.2)		0		(1.2)
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		(0.1)		0		(0.2)
RMBS	0		(0.1)		0		(0.8)
CDO/CLO	0		0		0		(0.2)
Other ABS	0		0		0		0
Net debt security impairments	0		(0.2)		0		(1.2)
Limited partnerships and other investment impairments	0		0		0		0
Net OTTI losses recognized in earnings	0		(0.2)		0		(1.2)
Debt security transaction gains	0.8		0.4		2.9		5.6
Debt security transaction losses	(0.4)		(0.2)		(1.1)		(0.3)
Limited partnerships and other investment transaction gains	0		0		0		0
Limited partnerships and other investment transaction losses	0		0		0		0
Net transaction gains (losses)	0.4		0.2		1.8		5.3
Derivative instruments	(2.1)		4.8		(25.7)		(16.1)
Embedded derivatives	(0.4)	[1]	5.9	[1]	(12.1)	[1]	5.3	[1]
Net realized investment gains (losses), excluding impairment losses	(2.1)		10.9		(36.0)		(5.5)
Net realized investment gains (losses), including impairment losses	$ (2.1)		$ 10.7		$ (36.0)		$ (6.7)

[1] Includes the change in fair value of embedded derivatives associated with fixed index annuity indexed crediting feature and variable annuity riders. See Note 7 to these interim unaudited financial statements for additional disclosures.