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, 2026	Mar. 31, 2025
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 483.4	$ 513.2
Net Investment Gain (Loss)	(5.0)	(206.8)
Interest and Debt Expense	53.1	52.0
Net Gain (Loss) on Derivatives	1.5	(5.3)
Designated as Hedging Instrument | Interest Rate Swaps | Income Statement Location: us-gaap:GainLossOnInvestments | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.0	(0.4)
Net Gain (Loss) on Derivatives	0.0	0.4
Designated as Hedging Instrument | Interest Rate Swaps | Income Statement Location: us-gaap:InterestAndDebtExpense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.7	0.7
Net Gain (Loss) on Derivatives	0.0	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Income Statement Location: us-gaap:NetInvestmentIncome | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	10.8	17.4
Net Gain (Loss) on Derivatives	1.3	2.6
Designated as Hedging Instrument | Foreign Exchange Contracts | Income Statement Location: us-gaap:GainLossOnInvestments | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(1.5)	0.0
Net Gain (Loss) on Derivatives	1.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Income Statement Location: us-gaap:GainLossOnInvestments | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	(7.0)	21.5
Net Gain (Loss) on Derivatives	7.0	(21.5)
Designated as Hedging Instrument | Foreign Exchange Contracts | Income Statement Location: us-gaap:InterestAndDebtExpense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.0	0.0
Net Gain (Loss) on Derivatives	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Income Statement Location: us-gaap:InterestAndDebtExpense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.0	0.0
Net Gain (Loss) on Derivatives	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Income Statement Location: us-gaap:NetInvestmentIncome | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	1.8	2.3
Net Gain (Loss) on Derivatives	0.1	(0.7)
Designated as Hedging Instrument | Foreign Exchange Contracts | Income Statement Location: us-gaap:NetInvestmentIncome | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	5.9	4.4
Net Gain (Loss) on Derivatives	2.8	(5.2)
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | Income Statement Location: us-gaap:GainLossOnInvestments | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Gain (Loss)	0.0	0.0
Net Gain (Loss) on Derivatives	0.0	0.0
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | Income Statement Location: us-gaap:InterestAndDebtExpense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0.0	0.0
Net Gain (Loss) on Derivatives	0.0	0.0
Designated as Hedging Instrument | Forward Benchmark Interest Rate Lock | Income Statement Location: us-gaap:NetInvestmentIncome | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	16.7	12.3
Net Gain (Loss) on Derivatives	$ (0.5)	$ (0.4)