Investing Activities - Net Realized Investment Gains/Losses (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Investments, Debt and Equity Securities [Abstract]
Total other-than-temporary debt impairments	$ (0.2)		$ (0.7)		$ (0.5)		$ (4.4)
Portion of loss recognized in OCI	0		(0.3)		(0.9)		1.8
Net debt impairments recognized in earnings	(0.2)		(1.0)		(1.4)		(2.6)
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		0		(0.2)		(0.1)
RMBS	(0.2)		(0.8)		(1.0)		(2.1)
CDO/CLO	0		(0.1)		(0.2)		(0.1)
Other asset-backed	0		(0.1)		0		(0.3)
Net debt security impairments	(0.2)		(1.0)		(1.4)		(2.6)
Equity security impairments	0		0		0		0
Net OTTI losses recognized in earnings	(0.2)		(1.0)		(1.4)		(2.6)
Debt security transaction gains	3.0		20.8		8.6		21.4
Debt security transaction losses	0		(0.1)		(0.3)		(0.2)
Limited partnerships and other investment gains	0		0		0		0
Limited partnerships and other investment losses	0		(0.3)		0		(0.4)
Net transaction gains (losses)	3.0		20.4		8.3		20.8
Derivative instruments	(14.0)		(18.0)		(30.1)		(33.5)
Embedded derivatives	8.4	[1]	6.6	[1]	13.7	[1]	6.1	[1]
Related party reinsurance derivatives	0		(4.2)		0		(3.5)
Net realized investment gains (losses), excluding impairment losses	(2.6)		4.8		(8.1)		(10.1)
Net realized investment gains (losses), including impairment losses	$ (2.8)		$ 3.8		$ (9.5)		$ (12.7)

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