Derivative Instruments - Gains (Losses) Recognized in Realized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss)	$ (6.9)	$ (2.3)	$ (20.4)	$ (38.8)
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss)	(4.3)	1.8	(1.6)	6.8
Variance swaps
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss)	(0.4)	(0.4)	(0.8)	(1.0)
Swaptions
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss)	0.0	(7.6)	(0.1)	(25.9)
Put options
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss)	(3.5)	(4.1)	(7.8)	(8.2)
Call options
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss)	(2.9)	17.2	(4.3)	14.2
Equity futures
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss)	0.0	(6.9)	0.2	(9.5)
Embedded derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss)	$ 4.2	$ (2.3)	$ (6.0)	$ (15.2)