Separate Accounts, Death Benefits and Other Insurance Benefit Features and Embedded Product Derivatives - GMDB and GMIB Benefits by Type (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	$ 2,269.4	$ 2,232.4
NAR after Reinsurance	21.1	34.9
Average Attained Age of Annuitant	64 years	64 years
Less: General account value with GMDB	306.0	311.3
Subtotal separate account liabilities with GMDB	1,963.4	1,921.1
Separate account liabilities without GMDB	146.1	140.7
Total separate account liabilities	2,109.5	2,061.8
GMIB	409.7	403.3
GMDB return of premium
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	761.6	755.9
NAR after Reinsurance	3.1	5.7
Average Attained Age of Annuitant	62 years	62 years
GMDB step up
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	1,443.6	1,412.4
NAR after Reinsurance	11.8	21.7
Average Attained Age of Annuitant	63 years	62 years
GMDB earnings enhancement benefit (EEB)
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	37.5	37.4
NAR after Reinsurance	0	0.1
Average Attained Age of Annuitant	63 years	63 years
GMDB greater of annual step up and roll up
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	26.7	26.7
NAR after Reinsurance	$ 6.2	$ 7.4
Average Attained Age of Annuitant	67 years	67 years