Derivative Instruments - Fair Value Derivatives (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Fair Value Assets	$ 170.0		$ 237.8
Fair Value Liabilities	85.0		110.8
Interest rate swaps
Derivatives, Fair Value [Line Items]
Notional Amount	114.0		139.0
Fair Value Assets	6.7		3.9
Fair Value Liabilities	3.2	[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	8.9	[1]	7.9	[1]
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount	3,277.0		3,902.0
Fair Value Assets	4.9		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	21.3		29.5
Fair Value Liabilities	0	[1]	0	[1]
Call options
Derivatives, Fair Value [Line Items]
Notional Amount	1,889.4		1,701.6
Fair Value Assets	126.3		161.2
Fair Value Liabilities	72.9	[1]	96.1	[1]
Equity futures
Derivatives, Fair Value [Line Items]
Notional Amount	135.6		159.7
Fair Value Assets	10.8		12.5
Fair Value Liabilities	0	[1]	0	[1]
Total derivative instruments
Derivatives, Fair Value [Line Items]
Notional Amount	5,807.9		6,294.2
Fair Value Assets	170.0		237.8
Fair Value Liabilities	$ 85.0	[1]	$ 110.8	[1]

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