Derivative Instruments - Fair Value Derivatives (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Fair Value Assets	$ 190.7		$ 149.4
Fair Value Liabilities	82.7		45.7
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	160.0		180.0
Fair Value Assets	6.8		15.5
Fair Value Liabilities	7.2	[1]	7.7	[1]
Variance Swap [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0.9		0.9
Fair Value Assets	0		0
Fair Value Liabilities	7.3	[1]	4.4	[1]
Swaptions [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,402.0		25.0
Fair Value Assets	22.2		0
Fair Value Liabilities	0	[1]	0	[1]
Put Option [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	391.0		391.0
Fair Value Assets	41.0		69.5
Fair Value Liabilities	0	[1]	0	[1]
Call Option [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,623.9		1,328.4
Fair Value Assets	105.4		50.6
Fair Value Liabilities	68.2	[1]	33.6	[1]
Equity futures [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	155.8		182.9
Fair Value Assets	15.3		13.8
Fair Value Liabilities	0	[1]	0	[1]
Total derivative instruments [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	3,733.6		2,108.2
Fair Value Assets	190.7		149.4
Fair Value Liabilities	$ 82.7	[1]	$ 45.7	[1]

[1] Derivative liabilities are included in other liabilities on the balance sheets.