Derivative Instruments - Gains (Losses) Recognized in Realized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ (13.5)	$ (36.5)
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	2.7	5.0
Variance swaps
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(0.4)	(0.6)
Swaptions
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(0.1)	(18.3)
Put options
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(4.3)	(4.1)
Call options
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(1.4)	(3.0)
Equity futures
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	0.2	(2.6)
Embedded Derivative Financial Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ (10.2)	$ (12.9)