Derivative Financial Instruments Location of Gains and Losses Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 476.8	$ 527.8	$ 1,550.7	$ 1,586.4
Net Investment Gain (Loss)	128.0	(12.9)	(96.5)	(24.5)
Interest and Debt Expense	52.2	49.2	156.2	148.6
Net Gain (Loss) on Derivatives	0.1	(2.2)	(3.1)	(0.2)
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	11.8	21.1	46.8	91.3
Net Gain (Loss) on Derivatives	1.5	3.7	7.0	14.5
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	39.2	0.0	38.9	0.0
Net Gain (Loss) on Derivatives	2.2	0.0	2.2	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.7	0.7	2.2	2.2
Net Gain (Loss) on Derivatives	0.1	0.0	0.1	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	1.6	2.3	5.9	6.5
Net Gain (Loss) on Derivatives	(0.9)	0.9	(1.3)	0.5
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	6.1	4.9	15.7	13.4
Net Gain (Loss) on Derivatives	3.7	4.2	1.4	11.5
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(0.9)	(0.7)	(1.0)	(0.4)
Net Gain (Loss) on Derivatives	0.8	0.7	0.9	0.5
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(2.3)	25.2	69.8	5.6
Net Gain (Loss) on Derivatives	2.3	(25.2)	(69.8)	(5.6)
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 | Forward Benchmark Interest Rate Lock | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	14.6	10.7	40.5	29.6
Net Gain (Loss) on Derivatives	(0.4)	(0.3)	(1.2)	(0.7)
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.5	0.0	0.5	0.0
Net Gain (Loss) on Derivatives	(3.1)	0.0	(3.1)	0.0
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | 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