Derivative Financial Instruments - Net Realized Gains (Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Other net realized capital gains (losses)	$ (102.0)	$ (327.0)	$ (189.0)
Net investment income	2,792.0	2,669.0	2,641.0
Gain (loss) reclassified from accumulated other comprehensive income into income	(278.0)	141.0	(21.0)
Other Comprehensive Income (Loss) | Designated as Hedging Instrument | Cash Flow Hedging | Interest rate contracts
Derivatives, Fair Value [Line Items]
Amount of Gain or (Loss) Recognized on Other Comprehensive Income / Reclassified from Accumulated Other Comprehensive Income	1.0
Other Comprehensive Income (Loss) | Designated as Hedging Instrument | Cash Flow Hedging | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Amount of Gain or (Loss) Recognized on Other Comprehensive Income / Reclassified from Accumulated Other Comprehensive Income	0.0
Other Net Realized Capital Gains (Losses) | Within fixed maturity investments
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	3.0	(6.0)	(10.0)
Other Net Realized Capital Gains (Losses) | Within products
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	(6.0)	92.0	65.0
Other Net Realized Capital Gains (Losses) | Designated as Hedging Instrument | Cash Flow Hedging | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	0.0
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	(136.0)	(38.0)	1.0
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	2.0	4.0	(7.0)
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Equity contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	(32.0)	10.0	(31.0)
Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Credit contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	2.0	(2.0)	13.0
Policyholder Benefits | Within reinsurance agreements
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	(111.0)	81.0	(52.0)
Investment Income | Designated as Hedging Instrument | Cash Flow Hedging | Interest rate contracts
Derivatives, Fair Value [Line Items]
Amount of Gain or (Loss) Recognized on Other Comprehensive Income / Reclassified from Accumulated Other Comprehensive Income	0.0
Investment Income | Designated as Hedging Instrument | Cash Flow Hedging | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Amount of Gain or (Loss) Recognized on Other Comprehensive Income / Reclassified from Accumulated Other Comprehensive Income	11.0
Gain (loss) reclassified from accumulated other comprehensive income into income	11.0
Held for sale
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	(44.0)	26.0	(3.0)
Held for sale | Other Net Realized Capital Gains (Losses) | Within fixed maturity investments
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	(1.0)	(6.0)	(8.0)
Held for sale | Other Net Realized Capital Gains (Losses) | Within products
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	(134.0)	69.0	(87.0)
Held for sale | Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	0.0	4.0	1.0
Held for sale | Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	0.0	0.0	(1.0)
Held for sale | Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Equity contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	139.0	(75.0)	93.0
Held for sale | Other Net Realized Capital Gains (Losses) | Not Designated as Hedging Instrument | Credit contracts
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	1.0	(1.0)	4.0
Held for sale | Policyholder Benefits | Within reinsurance agreements
Derivatives, Fair Value [Line Items]
Gain (loss) reclassified from accumulated other comprehensive income into income	$ (49.0)	$ 35.0	$ (5.0)