Investments - Net Realized Capital Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ (54.3)	$ (90.6)	$ (176.1)	$ (186.1)
After-tax net realized capital gains (losses), after tax	(35.3)	(58.9)	(114.4)	(121.0)
Proceeds from sale of investments
Proceeds on sales	386.6	610.1	1,520.8	1,246.6
Gross gains	2.2	3.7	17.0	11.5
Gross losses	2.2	7.6	63.2	23.1
Derivatives
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(5.4)	31.8	69.7	(9.9)
Other investments
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(0.1)	(0.1)	0.1	(0.1)
Fixed Maturities, Available-for-sale, Including Securities Pledged [Member]
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(2.9)	(26.1)	(50.5)	(36.5)
Fixed maturities, at fair value using the fair value option
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(48.8)	(31.2)	(90.7)	(83.6)
Equity securities, available-for-sale
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	0.0	0.0	0.0	(0.3)
Embedded derivatives - fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(1.6)	0.7	(1.3)	(2.0)
Guaranteed benefit derivatives
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ 4.5	$ (65.7)	$ (103.4)	$ (53.7)