Derivative Financial Instruments Location of Gains and Losses Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 2,213.2	$ 2,360.7	$ 2,435.3
Net Investment Gain (Loss)	76.7	1,199.1	(23.2)
Interest and Debt Expense	185.0	188.2	177.4
Derivative, Gain (Loss) on Derivative, Net	(3.1)	2.5	0.1
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	220.4	286.1	294.6
Derivative, Gain (Loss) on Derivative, Net	64.6	75.9	74.3
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 Investment Gain (Loss)	2.7	397.7	(1.6)
Derivative, Gain (Loss) on Derivative, Net	2.0	32.0	9.3
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)		(0.6)	(4.5)
Derivative, Gain (Loss) on Derivative, Net		0.6	4.5
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	29.2	29.2	30.4
Derivative, Gain (Loss) on Derivative, Net	5.0	1.7	2.4
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense		10.1	14.3
Derivative, Gain (Loss) on Derivative, Net		(0.9)	2.5
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	13.0	12.1	14.8
Derivative, Gain (Loss) on Derivative, Net	1.8	2.5	(2.0)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	9.8	7.1	2.9
Derivative, Gain (Loss) on Derivative, Net	4.6	2.8	1.9
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(0.1)	(0.1)	1.4
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.1	(1.4)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(22.3)	23.3	3.8
Derivative, Gain (Loss) on Derivative, Net	22.3	(23.3)	(3.8)
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	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	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	0.0
Derivative, Gain (Loss) on Derivative, Net	$ 0.0	$ 0.0	$ 0.0