Derivative Instruments - Gains (Losses) Recognized in Realized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ (21.5)	$ (1.7)
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(0.8)	(0.8)
Variance Swap
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(2.1)	(5.6)
Swaptions
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(1.8)	(0.2)
Put Option
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(16.8)	(8.0)
Call Option
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	17.0	10.0
Equity futures
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(16.4)	(14.8)
Embedded Derivative Financial Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(0.6)	23.0
Related party reinsurance derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ 0	$ (5.3)