Derivative Instruments - Fair Value Derivatives (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Fair Value Assets	$ 135.4	$ 157.5
Fair Value Liabilities	77.1	85.6
Interest rate swaps
Derivatives, Fair Value [Line Items]
Notional Amount	149.0	114.0
Fair Value Assets	4.0	9.7
Fair Value Liabilities	0.4	1.9
Variance swaps
Derivatives, Fair Value [Line Items]
Notional Amount	0.9	0.9
Fair Value Assets	0	0
Fair Value Liabilities	9.0	8.6
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount		777.0
Fair Value Assets		0.2
Fair Value Liabilities		0
Put options
Derivatives, Fair Value [Line Items]
Notional Amount	677.5	677.5
Fair Value Assets	25.1	29.4
Fair Value Liabilities	0	0
Call options
Derivatives, Fair Value [Line Items]
Notional Amount	2,169.3	2,019.2
Fair Value Assets	106.3	118.2
Fair Value Liabilities	67.7	74.6
Equity futures
Derivatives, Fair Value [Line Items]
Notional Amount		2.9
Fair Value Assets		0
Fair Value Liabilities		0.5
Total derivative instruments
Derivatives, Fair Value [Line Items]
Notional Amount	2,996.7	3,591.5
Fair Value Assets	135.4	157.5
Fair Value Liabilities	$ 77.1	$ 85.6