Investments - Net Realized Capital Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ (65.0)	$ (24.2)	$ (95.5)	$ (68.1)
After-tax net realized capital gains (losses), after tax	(42.3)	(15.7)	(62.1)	(44.2)
Proceeds from sale of investments
Proceeds on sales	474.0	499.0	636.5	970.0
Gross gains	6.3	8.6	7.8	20.9
Gross losses	11.8	10.2	15.5	20.1
Derivatives
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(73.5)	4.6	(41.7)	(12.9)
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(2.5)	(0.8)	(2.7)	(1.3)
Product guarantees
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	57.5	(14.2)	12.0	(32.0)
Other investments
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(0.1)	0.0	0.0	0.0
Fixed maturities, available-for-sale, including securities pledged
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(7.6)	(3.2)	(10.4)	0.3
Fixed maturities, at fair value using the fair value option
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(38.5)	(10.9)	(52.4)	(23.5)
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ (0.3)	$ 0.3	$ (0.3)	$ 1.3