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, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 478.4	$ 560.7	$ 961.8	$ 1,073.9
Net Investment Gain (Loss)	(5.2)	(17.7)	(10.2)	(224.5)
Interest and Debt Expense	53.3	52.0	106.4	104.0
Interest Rate Swaps | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	10.2	17.6	21.0	35.0
Net Investment Gain (Loss)	0.0	0.1	0.0	(0.3)
Interest and Debt Expense	0.7	1.5	1.4	1.5
Foreign Exchange Contracts | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	1.7	2.0	3.5	4.3
Net Investment Gain (Loss)	(1.7)	(0.1)	(3.2)	(0.1)
Interest and Debt Expense	0.0	0.0	0.0	0.0
Foreign Exchange Contracts | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	6.4	5.2	12.3	9.6
Net Investment Gain (Loss)	(8.5)	50.6	(15.5)	72.1
Interest and Debt Expense	0.0	0.0	0.0	0.0
Forward Benchmark Interest Rate Lock | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	18.2	13.6	34.9	25.9
Net Investment Gain (Loss)	0.0	0.0	0.0	0.0
Interest and Debt Expense	0.0	0.0	0.0	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	1.2	2.9	2.5	5.5
Net Investment Gain (Loss)	0.0	(0.4)	0.0	0.0
Interest and Debt Expense	0.0	0.0	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.1	0.3	0.2	(0.4)
Net Investment Gain (Loss)	1.0	0.1	2.0	0.1
Interest and Debt Expense	0.0	0.0	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	3.2	2.9	6.0	(2.3)
Net Investment Gain (Loss)	8.5	(50.6)	15.5	(72.1)
Interest and Debt Expense	0.0	0.0	0.0	0.0
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	(0.6)	(0.4)	(1.1)	(0.8)
Net Investment Gain (Loss)	0.0	0.0	0.0	0.0
Interest and Debt Expense	$ 0.0	$ 0.0	$ 0.0	$ 0.0