Investments - Net Realized Capital Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ (64.2)	$ (65.0)	$ (121.8)	$ (95.5)
After-tax net realized capital gains (losses), after tax	(41.7)	(42.3)	(79.1)	(62.1)
Proceeds from sale of investments
Proceeds on sales	441.5	474.0	1,134.2	636.5
Gross gains	10.9	6.3	14.8	7.8
Gross losses	23.2	11.8	61.0	15.5
Derivatives
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	12.2	(73.5)	75.1	(41.7)
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(0.6)	(2.5)	0.3	(2.7)
Product guarantees
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(45.1)	57.5	(107.9)	12.0
Other investments
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	0.3	(0.1)	0.2	0.0
Fixed maturities, available-for-sale, including securities pledged
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(6.4)	(7.6)	(47.6)	(10.4)
Fixed maturities, at fair value using the fair value option
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(24.6)	(38.5)	(41.9)	(52.4)
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ 0.0	$ (0.3)	$ 0.0	$ (0.3)