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, 2023	Mar. 31, 2022
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 508.8	$ 527.2
Net Investment Gain (Loss)	0.1	(13.8)
Interest and Debt Expense	48.1	46.9
Derivative, Gain (Loss) on Derivative, Net	0.6	(1.8)
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	49.5	50.4
Derivative, Gain (Loss) on Derivative, Net	10.4	13.4
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	2.5	2.8
Derivative, Gain (Loss) on Derivative, Net	0.3	0.2
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	3.2	2.7
Derivative, Gain (Loss) on Derivative, Net	1.3	1.4
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)	3.2	(5.3)
Derivative, Gain (Loss) on Derivative, Net	(3.2)	5.3
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	0.7	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	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