Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Fair Value Assets	$ 157.5	$ 225.3
Fair Value Liabilities	85.6	116.1
Interest rate swaps
Derivative [Line Items]
Notional Amount	114.0	139.0
Fair Value Assets	9.7	3.9
Fair Value Liabilities	1.9	6.8
Variance swaps
Derivative [Line Items]
Notional Amount	0.9	0.9
Fair Value Assets	0	0
Fair Value Liabilities	8.6	7.9
Swaptions
Derivative [Line Items]
Notional Amount	777.0	3,902.0
Fair Value Assets	0.2	30.7
Fair Value Liabilities	0	0
Put options
Derivative [Line Items]
Notional Amount	677.5	391.0
Fair Value Assets	29.4	29.5
Fair Value Liabilities	0	0
Call options
Derivative [Line Items]
Notional Amount	2,019.2	1,701.6
Fair Value Assets	118.2	161.2
Fair Value Liabilities	74.6	96.1
Equity futures
Derivative [Line Items]
Notional Amount	2.9	159.7
Fair Value Assets	0	0
Fair Value Liabilities	0.5	5.3
Total derivative instruments
Derivative [Line Items]
Notional Amount	3,591.5	6,294.2
Fair Value Assets	157.5	225.3
Fair Value Liabilities	$ 85.6	$ 116.1