Derivative Financial Instruments - Net Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivatives, Fair Value [Line Items]
Net Investment Income	$ 385.0	$ 428.0	$ 824.0	$ 822.0
Other net realized capital gains (losses)	(5.0)	(1.0)	(90.0)	(5.0)
Derivative, Gain (Loss) on Derivative, Net	45.0	(100.0)	(51.0)	(170.0)
Interest rate contracts | Other Comprehensive Income (Loss) | Designated as Hedging Instrument | Cash flow hedges
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	1.0	1.0	2.0	2.0
Interest rate contracts | Investment Income | Designated as Hedging Instrument | Cash flow hedges
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0	0.0
Interest rate contracts | Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	3.0	(50.0)	45.0	(86.0)
Foreign exchange contracts | Other Comprehensive Income (Loss) | Designated as Hedging Instrument | Cash flow hedges
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(16.0)	16.0	61.0	8.0
Foreign exchange contracts | Investment Income | Designated as Hedging Instrument | Cash flow hedges
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	2.0	3.0	5.0	5.0
Net realized gains (losses) on derivatives	2.0	3.0	5.0	5.0
Foreign exchange contracts | Other Net Realized Capital Gains (Losses) | Designated as Hedging Instrument | Cash flow hedges
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.0	0.0	0.0	0.0
Foreign exchange contracts | Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	1.0	4.0	2.0
Equity contract | Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	1.0	0.0	0.0	1.0
Fixed maturities | Other Net Realized Capital Gains (Losses)
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	1.0	4.0	2.0
Product | Other Net Realized Capital Gains (Losses)
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	30.0	(13.0)	(113.0)	(13.0)
Reinsurance agreements | Interest Credited and Other Benefits to Contract Owners
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	(39.0)	23.0	(77.0)
Managed Custody Guarantees | Gain (Loss) on Investments
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	9.0	0.0	(17.0)	0.0
Credit contracts | Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 2.0	$ 0.0	$ 3.0	$ 1.0