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, 2023	Dec. 31, 2022	Dec. 31, 2021
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 2,096.7	$ 2,122.2	$ 2,213.2
Net Investment Gain (Loss)	(36.0)	(15.7)	76.7
Interest and Debt Expense	194.8	188.5	185.0
Derivative, Gain (Loss) on Derivative, Net	0.6	(2.6)	(3.1)
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	198.6	200.0	220.4
Derivative, Gain (Loss) on Derivative, Net	35.2	51.0	64.6
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.8	0.0	2.7
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	2.0
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	2.9	2.9	29.2
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	5.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	9.6	12.3	13.0
Derivative, Gain (Loss) on Derivative, Net	0.0	(0.8)	1.8
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	14.5	11.6	9.8
Derivative, Gain (Loss) on Derivative, Net	11.1	7.3	4.6
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.4	(2.3)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	(0.4)	1.8	(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)	18.7	(26.8)	(22.3)
Derivative, Gain (Loss) on Derivative, Net	(18.7)	26.8	22.3
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	18.5	0.5
Derivative, Gain (Loss) on Derivative, Net	(0.5)	0.0
Designated as Hedging Instrument | Forwards | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Designated as Hedging Instrument | Forwards | 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