Derivative Instruments - Fair Value Derivatives (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Fair Value Assets	$ 199.3	$ 149.4
Fair Value Liabilities	77.4	45.7
Interest Rate Swap
Derivatives, Fair Value [Line Items]
Notional Amount	180.0	180.0
Fair Value Assets	8.7	15.5
Fair Value Liabilities	6.6	7.7
Variance Swap
Derivatives, Fair Value [Line Items]
Notional Amount	0.9	0.9
Fair Value Assets	0	0
Fair Value Liabilities	6.2	4.4
Swaptions
Derivatives, Fair Value [Line Items]
Notional Amount	2,779.0	25.0
Fair Value Assets	26.7	0
Fair Value Liabilities	0	0
Put Option
Derivatives, Fair Value [Line Items]
Notional Amount	391.0	391.0
Fair Value Assets	50.1	69.5
Fair Value Liabilities	0	0
Call Option
Derivatives, Fair Value [Line Items]
Notional Amount	1,531.7	1,328.4
Fair Value Assets	97.4	50.6
Fair Value Liabilities	64.6	33.6
Equity futures
Derivatives, Fair Value [Line Items]
Notional Amount	159.9	182.9
Fair Value Assets	16.4	13.8
Fair Value Liabilities	0	0
Total derivative instruments
Derivatives, Fair Value [Line Items]
Notional Amount	5,042.5	2,108.2
Fair Value Assets	199.3	149.4
Fair Value Liabilities	$ 77.4	$ 45.7

Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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