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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 613.2	$ 599.4	$ 1,767.2	$ 1,819.0
Net Realized Investment Gain (Loss)	4.4	(26.2)	(105.8)	(32.4)
Interest and Debt Expense	49.4	46.1	142.6	130.8
Derivative, Gain (Loss) on Derivative, Net	0.6	0.1	2.3	0.7
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	72.1	74.5	218.1	222.0
Derivative, Gain (Loss) on Derivative, Net	18.6	18.8	57.5	54.4
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	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 | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	0.0	1.7	0.1	(1.2)
Derivative, Gain (Loss) on Derivative, Net	0.0	5.3	0.4	9.3
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	0.6	(0.6)	(0.6)	(4.2)
Derivative, Gain (Loss) on Derivative, Net	(0.6)	0.6	0.6	4.2
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	7.3	7.7	21.9	23.1
Derivative, Gain (Loss) on Derivative, Net	0.4	0.6	1.7	1.8
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	2.9	3.5	10.1	10.7
Derivative, Gain (Loss) on Derivative, Net	(0.6)	0.6	(0.9)	2.1
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	2.4	3.3	8.5	11.6
Derivative, Gain (Loss) on Derivative, Net	0.8	(0.3)	2.1	(0.9)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	1.8	0.6	5.0	1.5
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.5	2.0	1.2
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(0.1)	(0.8)	(0.1)	1.1
Derivative, Gain (Loss) on Derivative, Net	0.1	0.7	0.1	(1.0)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	9.7	(4.8)	6.8	(4.0)
Derivative, Gain (Loss) on Derivative, Net	(9.7)	4.8	(6.8)	4.0
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