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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 531.1	$ 559.0	$ 1,039.9	$ 1,086.2
Net Investment Gain (Loss)	0.9	(4.1)	1.0	(17.9)
Interest and Debt Expense	48.9	47.4	97.0	94.3
Derivative, Gain (Loss) on Derivative, Net	(0.6)	(3.7)	0.0	(5.5)
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	49.0	50.3	98.5	100.7
Derivative, Gain (Loss) on Derivative, Net	9.0	13.0	19.4	26.4
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.8	0.0	0.8	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	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	0.8	0.7	1.5	1.4
Derivative, Gain (Loss) on Derivative, Net	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	2.5	2.8	5.0	5.6
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.4	0.2	0.6
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	3.2	3.1	6.4	5.8
Derivative, Gain (Loss) on Derivative, Net	2.3	2.7	3.6	4.1
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	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	0.1	0.0	0.1	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	3.3	(27.6)	6.5	(32.9)
Derivative, Gain (Loss) on Derivative, Net	(3.3)	27.6	(6.5)	32.9
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
Derivative, Gain (Loss) on Derivative, Net	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
Derivative, Gain (Loss) on Derivative, Net	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	3.1	0.0	3.8	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	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	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	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
Derivative, Gain (Loss) on Derivative, Net	$ 0.0	$ 0.0	$ 0.0	$ 0.0