Derivative Financial Instruments - Notional and Fair Values (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative assets	$ 684.8	$ 589.8
Derivatives liabilities	475.4	333.2
Fixed maturities
Derivatives, Fair Value [Line Items]
Derivative assets	27.5	27.8
Derivatives liabilities	0	0
Fixed indexed annuities (FIA)
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	174.8	129.2
Reinsurance Agreements
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	1.0	(13.0)
Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	25,762.2	28,146.2
Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	181.3	181.3
Credit contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	384.0	384.0
Designated as Hedging Instrument | Cash flow hedges | Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	472.8	513.3
Designated as Hedging Instrument | Cash flow hedges | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	51.2	51.2
Not Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	25,289.4	27,632.9
Not Designated as Hedging Instrument | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	130.1	130.1
Not Designated as Hedging Instrument | Equity contract
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	16.0	14.0
Not Designated as Hedging Instrument | Credit contracts
Derivatives, Fair Value [Line Items]
Derivatives, Notional Amount	384.0	384.0
Derivatives | Designated as Hedging Instrument | Cash flow hedges | Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivative assets	105.7	104.4
Derivatives liabilities	0	0
Derivatives | Designated as Hedging Instrument | Cash flow hedges | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivative assets	10.9	7.7
Derivatives liabilities	0	0
Derivatives | Not Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivative assets	519.6	432.8
Derivatives liabilities	287.8	209.2
Derivatives | Not Designated as Hedging Instrument | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivative assets	15.1	10.6
Derivatives liabilities	10.9	7.7
Derivatives | Not Designated as Hedging Instrument | Equity contract
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	0.1	0.1
Derivatives | Not Designated as Hedging Instrument | Credit contracts
Derivatives, Fair Value [Line Items]
Derivative assets	6.0	6.5
Derivatives liabilities	0	0
Derivatives | Not Designated as Hedging Instrument | Managed custody guarantees
Derivatives, Fair Value [Line Items]
Derivative assets	0	0
Derivatives liabilities	$ 0.8	$ 0