Derivative Instruments - Fair Value Derivatives (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Notional Amount	$ 2,926.3	$ 3,591.5
Derivative instruments	99.1	157.5
Derivative liabilities	54.3	85.6
Interest rate swaps
Derivatives, Fair Value [Line Items]
Notional Amount	239.5	114.0
Derivative instruments	1.8	9.7
Derivative liabilities	2.9	1.9
Variance swaps
Derivatives, Fair Value [Line Items]
Notional Amount	0.9	0.9
Derivative instruments	0.0	0.0
Derivative liabilities	9.4	8.6
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount		777.0
Derivative instruments		0.2
Derivative liabilities		0.0
Put options
Derivatives, Fair Value [Line Items]
Notional Amount	391.0	677.5
Derivative instruments	20.7	29.4
Derivative liabilities	0.0	0.0
Call options
Derivatives, Fair Value [Line Items]
Notional Amount	2,285.3	2,019.2
Derivative instruments	76.5	118.2
Derivative liabilities	42.0	74.6
Equity futures
Derivatives, Fair Value [Line Items]
Notional Amount	9.6	2.9
Derivative instruments	0.1	0.0
Derivative liabilities	$ 0.0	$ 0.5