Derivative Instruments - Gains (Losses) Recognized in Realized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ (3.4)	$ (5.6)	$ (41.2)	$ (16.4)
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	1.2	(2.9)	6.0	(8.9)
Variance swaps
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	0.2	(1.2)	(0.8)	(3.0)
Swaptions
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(1.8)	(4.7)	(27.6)	8.9
Put options
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	4.8	(8.5)	(3.4)	(29.1)
Call options
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	2.5	10.9	16.5	30.6
Equity futures
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(4.7)	(7.6)	(14.2)	(28.6)
Embedded Derivative Financial Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ (5.6)	$ 8.4	$ (17.7)	$ 13.7