Derivative Financial Instruments - Notional and Fair Values (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative assets	$ 512.7	$ 446.6
Derivatives liabilities	346.8	360.1
Total Derivatives, Asset Fair Value	566.4	505.8
Total Derivatives, Liability Fair Value	469.2	597.3
Interest rate contracts | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	18,131.1	17,555.1
Interest rate contracts | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Derivative assets	292.9	269.4
Derivatives liabilities	328.5	306.4
Foreign exchange contracts | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	161.6	213.4
Foreign exchange contracts | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Derivative assets	0.4	0.7
Derivatives liabilities	18.3	32.4
Equity contract | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	14.5	0
Equity contract | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Derivative assets	0.4	0
Derivatives liabilities	0	0
Credit contracts | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	347.5	548.4
Maximum potential future net exposure on sale of credit default swaps	329.0	518.3
Credit contracts | Not Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Derivative assets	3.6	2.6
Derivatives liabilities	0	21.2
Managed custody guarantees | Not Designated as Hedging Instrument | Future Policy Benefits and Claims Reserves
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	0	1.0
Cash Flow Hedging | Interest rate contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	1,000.0	1,000.0
Cash Flow Hedging | Interest rate contracts | Designated as Hedging Instrument | Derivatives
Derivatives, Fair Value [Line Items]
Derivative assets	215.4	173.9
Derivatives liabilities	0	0
Fixed maturities | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative assets	53.7	59.2
Derivatives liabilities	0	0
Fixed indexed annuities (FIA) | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	$ 122.4	$ 236.3