Investments - Net Realized Capital Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Available-for-sale Securities, Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ (190.6)	$ (874.8)
After-tax net realized capital gains (losses), after tax	(118.3)	(568.7)
Proceeds from sale of investments
Proceeds on sales	1,494.9	3,212.7
Gross gains	39.7	42.0
Gross losses	19.8	14.4
Derivatives
Available-for-sale Securities, Including Securities Pledged [Line Items]
Realized capital gains (losses)	53.8	(1,099.7)
Fixed maturities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Realized capital gains (losses)	(3.3)	(23.3)
Product guarantees
Available-for-sale Securities, Including Securities Pledged [Line Items]
Realized capital gains (losses)	(255.1)	346.3
Other investments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Realized capital gains (losses)	2.1	(0.1)
Fixed maturities, available-for-sale, including securities pledged
Available-for-sale Securities, Including Securities Pledged [Line Items]
Realized capital gains (losses)	13.7	9.4
Fixed maturities, at fair value using the fair value option
Available-for-sale Securities, Including Securities Pledged [Line Items]
Realized capital gains (losses)	(18.9)	(107.6)
Equity securities, available-for-sale
Available-for-sale Securities, Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ 17.1	$ 0.2