Investing Activities - Net Realized Investment Gains/Losses (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Investments, Debt and Equity Securities [Abstract]
Total other-than-temporary debt impairments	$ (0.5)		$ (0.2)		$ (0.5)		$ (0.5)
Portion of gains (losses) recognized in OCI	0		0		0		(0.9)
Net debt impairments recognized in earnings	(0.5)		(0.2)		(0.5)		(1.4)
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	(0.5)		0		(0.5)		0
CMBS	0		0		0		(0.2)
RMBS	0		(0.2)		0		(1.0)
CDO/CLO	0		0		0		(0.2)
Other ABS	0		0		0		0
Net debt security impairments	(0.5)		(0.2)		(0.5)		(1.4)
Limited partnerships and other investment impairments	0		0		0		0
Net OTTI losses recognized in earnings	(0.5)		(0.2)		(0.5)		(1.4)
Debt security transaction gains	1.7		3.0		4.6		8.6
Debt security transaction losses	(1.0)		0		(2.1)		(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)	0.7		3.0		2.5		8.3
Derivative instruments	2.2		(14.0)		(23.5)		(30.1)
Embedded derivatives	(5.6)	[1]	8.4	[1]	(17.7)	[1]	13.7	[1]
Net realized investment gains (losses), excluding impairment losses	(2.7)		(2.6)		(38.7)		(8.1)
Net realized investment gains (losses), including impairment losses	$ (3.2)		$ (2.8)		$ (39.2)		$ (9.5)

[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.