Derivative Financial Instruments Location of Gains and Losses Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 2,032.7	$ 2,130.0	$ 2,096.7
Net Investment Loss	(106.6)	(34.6)	(36.0)
Interest and Debt Expense	208.8	201.1	194.8
Derivative, Gain (Loss) on Derivative, Net	2.9	(3.8)	0.6
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	58.6	109.6	198.6
Derivative, Gain (Loss) on Derivative, Net	8.4	17.4	35.2
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Loss	37.7	0.0	0.8
Derivative, Gain (Loss) on Derivative, Net	2.2	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	2.9	2.9	2.9
Derivative, Gain (Loss) on Derivative, Net	0.2	0.0	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	7.8	8.6	9.6
Derivative, Gain (Loss) on Derivative, Net	0.6	0.7	0.0
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	21.5	17.1	14.5
Derivative, Gain (Loss) on Derivative, Net	0.8	14.5	11.1
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Loss	(2.1)	(0.4)	0.4
Derivative, Gain (Loss) on Derivative, Net	2.5	0.4	(0.4)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Loss	71.0	(40.2)	18.7
Derivative, Gain (Loss) on Derivative, Net	(71.0)	40.2	(18.7)
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
Derivative, Gain (Loss) on Derivative, Net	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
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0
Designated as Hedging Instrument | Forwards | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	56.2	40.7	18.5
Derivative, Gain (Loss) on Derivative, Net	(1.7)	(1.0)	(0.5)
Designated as Hedging Instrument | Forwards | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Loss	0.5	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	(3.1)	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
Derivative, Gain (Loss) on Derivative, Net	$ 0.0	$ 0.0	$ 0.0