Derivative Financial Instruments - Net Realized Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Cash collateral held for securities loan agreement	$ 0				$ 0				$ 16.3
Net realized gains (losses) on derivatives	(28.5)		43.5		(87.6)		43.3
Other Net Realized Capital Gains (Losses) | Within fixed maturity investments
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	(0.8)	[1]	(11.8)	[1]	(1.3)	[1]	(17.2)	[1]
Other Net Realized Capital Gains (Losses) | Fixed indexed annuities (FIA)
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	(14.3)	[1]	52.1	[1]	(32.1)	[1]	76.4	[1]
Other Net Realized Capital Gains (Losses) | Designated as Hedging Instrument | Cash flow hedges | Interest rate contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	(0.1)	[2]	0	[2]	0	[2]	0.1	[2]
Other Net Realized Capital Gains (Losses) | Designated as Hedging Instrument | Cash flow hedges | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.2	[2]	0	[2]	0.3	[2]	0	[2]
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	3.0	[1]	(45.4)	[1]	(15.6)	[1]	(77.9)	[1]
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	(0.2)	[1]	2.6	[1]	0	[1]	12.0	[1]
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Equity contract
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.6	[1]	0.4	[1]	1.0	[1]	1.7	[1]
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Credit contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	1.1	[1]	0.5	[1]	1.4	[1]	2.3	[1]
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Managed custody guarantees
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.1	[1]	0.1	[1]	0.1	[1]	0.1	[1]
Interest Credited and Other Benefits to Contract Owners | Reinsurance Agreements
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	$ (18.1)	[3]	$ 45.0	[3]	$ (41.4)	[3]	$ 45.8	[3]

[1] Changes in value are included in Other net realized capital gains (losses) in the Condensed Consolidated Statements of Operations.

[2] Changes in value for effective fair value hedges are recorded in Other net realized capital gains (losses) in the Condensed Consolidated Statements of Operations. Changes in fair value upon disposal for effective cash flow hedges are amortized through Net investment income and the ineffective portion is recorded in the Other net realized capital gains (losses) in the Condensed Consolidated Statements of Operations. For the three and six months ended June 30, 2014 and 2013, ineffective amounts were immaterial.

[3] Changes in value are included in Interest credited and other benefits to contract owners/policyholders in the Condensed Consolidated Statements of Operations.