Derivative Financial Instruments - Notional and Fair Values (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative assets	$ 527.3	$ 566.4
Derivatives liabilities	395.3	469.2
Fixed maturities
Derivatives, Fair Value [Line Items]
Derivative assets	48.2	53.7
Derivatives liabilities	0	0
Fixed indexed annuities (FIA)
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	99.6	122.4
Reinsurance Agreements
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	(0.8)	0
Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	19,278.8	19,131.1
Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	145.8	161.6
Equity contract
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	0	0
Credit contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	344.0	347.5
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	937.5	1,000.0
Not Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	18,341.3	18,131.1
Not Designated as Hedging Instrument | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	145.8	161.6
Not Designated as Hedging Instrument | Equity contract
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	9.2	14.5
Not Designated as Hedging Instrument | Credit contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	344.0	347.5
Maximum potential future net exposure on sale of credit default swaps	329.0	329.0
Derivatives | Designated as Hedging Instrument | Cash Flow Hedging | Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivative assets	188.1	215.4
Derivatives liabilities	0	0
Derivatives | Not Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivative assets	283.4	292.9
Derivatives liabilities	284.4	328.5
Derivatives | Not Designated as Hedging Instrument | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivative assets	3.0	0.4
Derivatives liabilities	12.1	18.3
Derivatives | Not Designated as Hedging Instrument | Equity contract
Derivatives, Fair Value [Line Items]
Derivative assets	0	0.4
Derivatives liabilities	0	0
Derivatives | Not Designated as Hedging Instrument | Credit contracts
Derivatives, Fair Value [Line Items]
Derivative assets	4.6	3.6
Derivatives liabilities	0	0
Derivatives | Not Designated as Hedging Instrument | Managed custody guarantees
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	$ 0	$ 0