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, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 527.8	$ 526.0	$ 1,586.4	$ 1,565.9
Net Investment Gain (Loss)	(12.9)	(31.0)	(24.5)	(30.0)
Interest and Debt Expense	49.2	48.6	148.6	145.6
Net Gain (Loss) on Derivatives	(2.2)	2.0	(0.2)	2.0
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	21.1	49.7	91.3	148.2
Net Gain (Loss) on Derivatives	3.7	8.3	14.5	27.7
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	0.0	0.8
Net Gain (Loss) on Derivatives	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	0.7	0.7	2.2	2.2
Net Gain (Loss) on Derivatives	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	2.3	2.3	6.5	7.3
Net Gain (Loss) on Derivatives	0.9	0.4	0.5	0.6
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	4.9	3.9	13.4	10.3
Net Gain (Loss) on Derivatives	4.2	3.1	11.5	6.7
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(0.7)	(0.1)	(0.4)	(0.1)
Net Gain (Loss) on Derivatives	0.7	0.0	0.5	0.1
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	25.2	(15.2)	5.6	(8.7)
Net Gain (Loss) on Derivatives	(25.2)	15.2	(5.6)	8.7
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 | Forwards | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	10.7	3.9	29.6	7.7
Net Gain (Loss) on Derivatives	(0.3)	(0.1)	(0.7)	(0.1)
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
Net Gain (Loss) on Derivatives	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
Net Gain (Loss) on Derivatives	$ 0.0	$ 0.0	$ 0.0	$ 0.0