Derivative Financial Instruments Location of Gains and Losses Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 513.5	$ 508.8
Net Investment Gain (Loss)	(1.2)	0.1
Interest and Debt Expense	49.5	48.1
Derivative, Gain (Loss) on Derivative, Net	(1.6)	0.6
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	47.6	49.5
Derivative, Gain (Loss) on Derivative, Net	6.0	10.4
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	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	0.7	0.7
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	2.2	2.5
Derivative, Gain (Loss) on Derivative, Net	(0.5)	0.3
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	4.2	3.2
Derivative, Gain (Loss) on Derivative, Net	3.1	1.3
Designated as Hedging Instrument | Foreign Exchange Contracts | Net 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 | Foreign Exchange Contracts | Net Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(17.6)	3.2
Derivative, Gain (Loss) on Derivative, Net	17.6	(3.2)
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
Derivative, Gain (Loss) on Derivative, Net	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
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Designated as Hedging Instrument | Forwards | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	9.0	0.7
Derivative, Gain (Loss) on Derivative, Net	0.2	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
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