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, 2022	Dec. 31, 2021	Dec. 31, 2020
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 2,122.2	$ 2,213.2	$ 2,360.7
Net Investment Gain (Loss)	(15.7)	76.7	1,199.1
Interest and Debt Expense	188.5	185.0	188.2
Derivative, Gain (Loss) on Derivative, Net	(2.6)	(3.1)	2.5
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	200.0	220.4	286.1
Derivative, Gain (Loss) on Derivative, Net	51.0	64.6	75.9
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income			0.0
Derivative, Gain (Loss) on Derivative, Net			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)	0.0	2.7	397.7
Derivative, Gain (Loss) on Derivative, Net	0.0	2.0	32.0
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)
Derivative, Gain (Loss) on Derivative, Net			0.6
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	2.9	29.2	29.2
Derivative, Gain (Loss) on Derivative, Net	0.0	5.0	1.7
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense			10.1
Derivative, Gain (Loss) on Derivative, Net			(0.9)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	12.3	13.0	12.1
Derivative, Gain (Loss) on Derivative, Net	(0.8)	1.8	2.5
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	11.6	9.8	7.1
Derivative, Gain (Loss) on Derivative, Net	7.3	4.6	2.8
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(2.3)	(0.1)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	1.8	(0.1)	0.1
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(26.8)	(22.3)	23.3
Derivative, Gain (Loss) on Derivative, Net	26.8	22.3	(23.3)
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
Designated as Hedging Instrument | Forwards | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.5
Designated as Hedging Instrument | Forwards | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.0
Designated as Hedging Instrument | Forwards | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	$ 0.0