Derivative Instruments - Gains (Losses) Recognized in Realized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ 10.7	$ (13.6)	$ (10.8)	$ (15.3)
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(5.2)	1.5	(6.0)	0.7
Variance Swap
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	0.3	0.9	(1.8)	(4.7)
Swaptions
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	15.4	0	13.6	(0.2)
Put Option
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(3.8)	6.2	(20.6)	(1.8)
Call Option
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	2.7	(9.9)	19.7	0.1
Equity futures
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(4.6)	5.2	(21.0)	(9.6)
Embedded Derivative Financial Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	5.9	(23.5)	5.3	(0.5)
Related party reinsurance derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ 0	$ 6.0	$ 0	$ 0.7