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, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 560.7	$ 545.1	$ 1,073.9	$ 1,058.6
Net Investment Gain (Loss)	(17.7)	(10.4)	(224.5)	(11.6)
Interest and Debt Expense	52.0	49.9	104.0	99.4
Net Gain (Loss) on Derivatives	2.1	0.4	(3.2)	2.0
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	17.6	22.6	35.0	70.2
Net Gain (Loss) on Derivatives	2.9	4.8	5.5	10.8
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.1	0.0	(0.3)	0.0
Net Gain (Loss) on Derivatives	(0.4)	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	1.5	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	2.0	4.3	4.2
Net Gain (Loss) on Derivatives	0.3	0.1	(0.4)	(0.4)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	5.2	4.3	9.6	8.5
Net Gain (Loss) on Derivatives	2.9	4.2	(2.3)	7.3
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(0.1)	0.3	(0.1)	0.3
Net Gain (Loss) on Derivatives	0.1	(0.2)	0.1	(0.2)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	50.6	(2.0)	72.1	(19.6)
Net Gain (Loss) on Derivatives	(50.6)	2.0	(72.1)	19.6
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	13.6	9.9	25.9	18.9
Net Gain (Loss) on Derivatives	(0.4)	(0.2)	(0.8)	(0.4)
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