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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 526.0	$ 511.6	$ 1,565.9	$ 1,597.8
Net Investment Gain (Loss)	(31.0)	(4.4)	(30.0)	(22.3)
Interest and Debt Expense	48.6	47.0	145.6	141.3
Derivative, Gain (Loss) on Derivative, Net	(2.0)	0.9	(2.0)	(4.6)
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	49.7	50.2	148.2	150.9
Derivative, Gain (Loss) on Derivative, Net	8.3	14.1	27.7	40.5
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	0.8	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	0.7	0.7	2.2	2.1
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	2.3	2.7	7.3	8.3
Derivative, Gain (Loss) on Derivative, Net	0.4	(1.5)	0.6	(0.9)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	3.9	2.9	10.3	8.7
Derivative, Gain (Loss) on Derivative, Net	3.1	1.0	6.7	5.1
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(0.1)	(0.5)	(0.1)	(0.5)
Derivative, Gain (Loss) on Derivative, Net	0.0	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)	(15.2)	(29.6)	(8.7)	(62.5)
Derivative, Gain (Loss) on Derivative, Net	15.2	29.6	8.7	62.5
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	3.9	0.2	7.7	0.2
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.0	(0.1)	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