Investments - Net Realized Capital Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ (90.6)	$ (42.1)	$ (186.1)	$ (110.2)
After-tax net realized capital gains (losses), after tax	(58.9)	(27.4)	(121.0)	(71.6)
Proceeds from sale of investments
Proceeds on sales	610.1	308.7	1,246.6	1,278.7
Gross gains	3.7	1.3	11.5	22.2
Gross losses	7.6	6.9	23.1	27.0
Derivatives
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	31.8	11.5	(9.9)	(1.4)
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	0.7	0.4	(2.0)	(0.9)
Product guarantees
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(65.7)	(12.5)	(53.7)	(44.5)
Other investments
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(0.1)	0.0	(0.1)	0.0
Fixed maturities, available-for-sale, including securities pledged
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(26.1)	(6.8)	(36.5)	(6.5)
Fixed maturities, at fair value using the fair value option
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(31.2)	(34.7)	(83.6)	(58.2)
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ 0.0	$ 0.0	$ (0.3)	$ 1.3