Derivative Instruments Derivative Instrument Gains (Losses) Recognized in Realized Investment Income (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ (67.3)	$ (12.1)	$ (53.0)
Interest rate swaps
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	11.0	(11.4)	(1.7)
Variance swaps
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(0.7)	(3.6)	(7.9)
Swaptions
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(30.5)	17.3	(0.2)
Put options
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(4.9)	(40.7)	(20.9)
Call options
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	18.8	60.1	0.9
Equity futures
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(15.6)	(44.8)	(19.2)
Embedded derivatives
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	(45.4)	11.0	4.0
Related party reinsurance derivatives
Derivative [Line Items]
Total derivative instrument gains (losses) recognized in realized investment gains (losses)	$ 0	$ 0	$ (8.0)