Separate Accounts, Death Benefits and Other Insurance Benefit Features and Embedded Product Derivatives - Changes in Guaranteed Insurance Benefit Liability Balances (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Mar. 31, 2012
Variable Annuity GMDB [Member]
Movement in Guaranteed Benefit Liability, Gross [Roll Forward]
Liability balance, beginning of the period	$ 10.8	$ 10.8	$ 10.8
Incurred	1.8	1.0
Paid	(2.4)	(1.0)
Liabilities for Guarantees on Long-Duration Contracts, Net Unrealized Gains (Losses) included in Accumulated Other Comprehensive Income	0	0
Assumption unlocking	0	0
Liability balance, end of the period	10.2	10.8	10.8
Variable Annuity GMIB [Member]
Movement in Guaranteed Benefit Liability, Gross [Roll Forward]
Liability balance, beginning of the period	20.9	17.0	20.9
Incurred	(2.2)	3.8
Paid	0	0
Liabilities for Guarantees on Long-Duration Contracts, Net Unrealized Gains (Losses) included in Accumulated Other Comprehensive Income	(0.1)	0.3
Assumption unlocking	0	(0.2)
Liability balance, end of the period	18.6	20.9	20.9
Fixed Indexed Annuity GMWB & GMDB [Member]
Movement in Guaranteed Benefit Liability, Gross [Roll Forward]
Liability balance, beginning of the period	103.6	5.6	103.6
Incurred	44.9	40.1
Paid	(0.2)	0
Liabilities for Guarantees on Long-Duration Contracts, Net Unrealized Gains (Losses) included in Accumulated Other Comprehensive Income	(47.7)	57.9
Assumption unlocking	0	0
Liability balance, end of the period	100.6	103.6	103.6
Universal Life Secondary Guarantees [Member]
Movement in Guaranteed Benefit Liability, Gross [Roll Forward]
Liability balance, beginning of the period	115.8	100.6	115.8
Incurred	31.9	22.6
Paid	(11.8)	(9.5)
Assumption unlocking	0	0
Liability balance, end of the period	134.1	115.8	115.8
Universal Life [Member]
Movement in Guaranteed Benefit Liability, Gross [Roll Forward]
Liabilities for Guarantees on Long-Duration Contracts, Net Unrealized Gains (Losses) included in Accumulated Other Comprehensive Income	$ (1.8)	$ 2.1