Investments - Net Realized Capital Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ (47.5)	$ (64.2)	$ (102.8)	$ (121.8)
After-tax net realized capital gains (losses), after tax	(30.8)	(41.7)	(66.8)	(79.1)
Proceeds from sale of investments
Proceeds on sales	560.8	441.5	1,637.9	1,134.2
Gross gains	5.3	10.9	10.7	14.8
Gross losses	6.7	23.2	24.7	61.0
Derivatives
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	2.4	12.2	6.3	75.1
Other investments
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	0.3	0.3	0.2	0.2
Fixed Maturities, Available-for-sale, Including Securities Pledged [Member]
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(1.6)	(6.4)	(20.7)	(47.6)
Fixed maturities, at fair value using the fair value option
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(38.3)	(24.6)	(97.1)	(41.9)
Embedded derivatives - fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	(1.0)	(0.6)	(2.5)	0.3
Guaranteed benefit derivatives
Available-for-sale Securities Including Securities Pledged [Line Items]
Realized capital gains (losses)	$ (9.3)	$ (45.1)	$ 11.0	$ (107.9)