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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 599.4	$ 619.2	$ 1,819.0	$ 1,845.1
Net Realized Investment Gain (Loss)	(26.2)	6.7	(32.4)	1.9
Interest and Debt Expense	46.1	42.7	130.8	125.3
Derivative, Gain (Loss) on Derivative, Net	0.1	(0.2)	0.7	(0.9)
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	74.5	75.8	222.0	230.3
Derivative, Gain (Loss) on Derivative, Net	18.8	16.7	54.4	49.1
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.0	0.0	0.0	1.0
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0	(0.5)
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	1.7	0.0	(1.2)	0.1
Derivative, Gain (Loss) on Derivative, Net	5.3	0.0	9.3	(0.3)
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(0.6)	0.0	(4.2)	2.2
Derivative, Gain (Loss) on Derivative, Net	0.6	0.0	4.2	(2.2)
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	7.7	8.5	23.1	31.3
Derivative, Gain (Loss) on Derivative, Net	0.6	0.6	1.8	1.7
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	3.5	3.6	10.7	10.8
Derivative, Gain (Loss) on Derivative, Net	0.6	0.6	2.1	1.2
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	3.3	4.6	11.6	14.3
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(0.3)	(0.9)	(0.8)
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.6	0.1	1.5	0.5
Derivative, Gain (Loss) on Derivative, Net	0.5	0.1	1.2	0.2
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(0.8)	0.4	1.1	1.3
Derivative, Gain (Loss) on Derivative, Net	0.7	(0.4)	(1.0)	(1.3)
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(4.8)	(0.3)	(4.0)	(1.7)
Derivative, Gain (Loss) on Derivative, Net	4.8	0.3	4.0	1.7
Foreign Exchange Contracts | Interest Expense | Designated as Hedging Instrument | 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
Foreign Exchange Contracts | Interest Expense | Designated as Hedging Instrument | 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