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	6 Months Ended	12 Months Ended		6 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended
	Jun. 30, 2013 Variable Annuity GMIB [Member]	Dec. 31, 2012 Variable Annuity GMIB [Member]	Mar. 31, 2012 Variable Annuity GMIB [Member]	Jun. 30, 2013 Variable Annuity GMDB [Member]	Dec. 31, 2012 Variable Annuity GMDB [Member]	Mar. 31, 2012 Variable Annuity GMDB [Member]	Jun. 30, 2013 Fixed Indexed Annuity GMWB & GMDB [Member]	Dec. 31, 2012 Fixed Indexed Annuity GMWB & GMDB [Member]	Mar. 31, 2012 Fixed Indexed Annuity GMWB & GMDB [Member]	Jun. 30, 2013 Universal Life [Member]	Dec. 31, 2012 Universal Life [Member]	Mar. 31, 2012 Universal Life [Member]	Jun. 30, 2013 Universal Life [Member]	Dec. 31, 2012 Universal Life [Member]
Movement in Guaranteed Benefit Liability, Gross [Roll Forward]
Liability balance, beginning of the period	$ 20.9	$ 17.0	$ 20.9	$ 10.8	$ 10.8	$ 10.8	$ 89.4	$ 5.6	$ 103.6	$ 131.6	$ 100.6	$ 115.8	$ 300.4	$ 203.0
Incurred	0	3.8		1.2	1.0		32.8	40.1		22.8	22.6		32.0	37.1
Paid	0	0		(1.6)	(1.0)		(0.2)	0		(5.3)	(9.5)
Liabilities for Guarantees on Long-Duration Contracts, Net Unrealized Gains (Losses) included in Accumulated Other Comprehensive Income	(0.1)	0.3		0	0		(46.8)	57.9		(1.7)	2.1		(6.4)	16.9
Assumption unlocking	0	(0.2)		0	0		0	0		0	0		0	43.4
Liability balance, end of the period	$ 20.8	$ 20.9	$ 20.9	$ 10.4	$ 10.8	$ 10.8		$ 103.6	$ 103.6		$ 115.8	$ 115.8	$ 326.0	$ 300.4