Derivative Financial Instruments Location of Gains and Losses Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 548.7	$ 585.0
Net Realized Investment Gain (Loss)	84.6	(144.0)
Interest and Debt Expense	44.4	45.7
Derivative, Gain (Loss) on Derivative, Net	(1.7)	0.7
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	52.0	73.0
Derivative, Gain (Loss) on Derivative, Net	15.5	19.2
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	2.4	0.0
Derivative, Gain (Loss) on Derivative, Net	1.8	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	0.0	(1.5)
Derivative, Gain (Loss) on Derivative, Net	0.0	1.5
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	7.3	7.3
Derivative, Gain (Loss) on Derivative, Net	0.1	0.6
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.0	3.6
Derivative, Gain (Loss) on Derivative, Net	0.0	0.1
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	3.1	2.9
Derivative, Gain (Loss) on Derivative, Net	0.3	0.9
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	2.3	1.4
Derivative, Gain (Loss) on Derivative, Net	1.0	0.8
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(9.5)	(13.2)
Derivative, Gain (Loss) on Derivative, Net	9.5	13.2
Designated as Hedging Instrument | Foreign Exchange Contracts | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Interest and Debt Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	$ 0.0	$ 0.0