Derivative Instruments - Fair Value Derivatives (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Fair Value Assets	$ 167.6		$ 237.8
Fair Value Liabilities	84.6		110.8
Interest rate swaps
Derivatives, Fair Value [Line Items]
Notional Amount	94.0		139.0
Fair Value Assets	5.6		3.9
Fair Value Liabilities	3.8	[1]	6.8	[1]
Variance swaps
Derivatives, Fair Value [Line Items]
Notional Amount	0.9		0.9
Fair Value Assets	0		0
Fair Value Liabilities	8.5	[1]	7.9	[1]
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount	3,277.0		3,902.0
Fair Value Assets	12.5		30.7
Fair Value Liabilities	0	[1]	0	[1]
Put options
Derivatives, Fair Value [Line Items]
Notional Amount	391.0		391.0
Fair Value Assets	25.4		29.5
Fair Value Liabilities	0	[1]	0	[1]
Call options
Derivatives, Fair Value [Line Items]
Notional Amount	1,811.4		1,701.6
Fair Value Assets	113.1		161.2
Fair Value Liabilities	72.3	[1]	96.1	[1]
Equity futures
Derivatives, Fair Value [Line Items]
Notional Amount	156.4		159.7
Fair Value Assets	11.0		12.5
Fair Value Liabilities	0	[1]	0	[1]
Total derivative instruments
Derivatives, Fair Value [Line Items]
Notional Amount	5,730.7		6,294.2
Fair Value Assets	167.6		237.8
Fair Value Liabilities	$ 84.6	[1]	$ 110.8	[1]

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