Derivative Financial Instruments Location of Gains and Losses Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 545.1	$ 531.1	$ 1,058.6	$ 1,039.9
Net Investment Gain (Loss)	(10.4)	0.9	(11.6)	1.0
Interest and Debt Expense	49.9	48.9	99.4	97.0
Net Gain (Loss) on Derivatives	0.4	0.6	2.0	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	22.6	49.0	70.2	98.5
Net Gain (Loss) on Derivatives	4.8	9.0	10.8	19.4
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.0	0.8	0.0	0.8
Net Gain (Loss) on Derivatives	0.0	0.0	0.0	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.8	0.8	1.5	1.5
Net Gain (Loss) on Derivatives	0.0	0.0	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	2.0	2.5	4.2	5.0
Net Gain (Loss) on Derivatives	0.1	(0.1)	(0.4)	0.2
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	4.3	3.2	8.5	6.4
Net Gain (Loss) on Derivatives	4.2	2.3	7.3	3.6
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.3	0.0	0.3	0.0
Net Gain (Loss) on Derivatives	(0.2)	0.1	(0.2)	0.1
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(2.0)	3.3	(19.6)	6.5
Net Gain (Loss) on Derivatives	2.0	(3.3)	19.6	(6.5)
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	0.0
Net Gain (Loss) on Derivatives	0.0	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	0.0
Net Gain (Loss) on Derivatives	0.0	0.0	0.0	0.0
Designated as Hedging Instrument | Forwards | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	9.9	3.1	18.9	3.8
Net Gain (Loss) on Derivatives	(0.2)	0.0	(0.4)	0.0
Designated as Hedging Instrument | Forwards | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.0	0.0	0.0	0.0
Net Gain (Loss) on Derivatives	0.0	0.0	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	0.0	0.0
Net Gain (Loss) on Derivatives	$ 0.0	$ 0.0	$ 0.0	$ 0.0