Derivative Financial Instruments Location of Gains and Losses Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 624.9	$ 623.6	$ 1,219.6	$ 1,225.9
Net Realized Investment Gain (Loss)	(7.3)	(2.6)	(6.2)	(4.8)
Interest and Debt Expense	42.6	42.4	84.7	82.6
Derivative, Gain (Loss) on Derivative, Net	0.9	0.0	0.6	(0.7)
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	75.1	76.7	147.5	154.5
Derivative, Gain (Loss) on Derivative, Net	17.9	16.5	35.6	32.4
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.0	0.3	0.0	1.0
Derivative, Gain (Loss) on Derivative, Net	0.0	(0.1)	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)	0.6	0.0	(2.9)	0.1
Derivative, Gain (Loss) on Derivative, Net	0.0	(0.2)	4.0	(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)	(2.1)	0.3	(3.6)	2.2
Derivative, Gain (Loss) on Derivative, Net	2.1	(0.3)	3.6	(2.2)
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	7.7	11.4	15.4	22.8
Derivative, Gain (Loss) on Derivative, Net	0.6	0.5	1.2	1.1
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	3.6	3.6	7.2	7.2
Derivative, Gain (Loss) on Derivative, Net	0.7	0.5	1.5	0.6
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	3.7	4.7	8.3	9.7
Derivative, Gain (Loss) on Derivative, Net	(0.4)	(0.2)	(0.6)	(0.5)
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.5	0.3	0.9	0.4
Derivative, Gain (Loss) on Derivative, Net	0.4	0.1	0.7	0.1
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	1.1	0.9	1.9	0.9
Derivative, Gain (Loss) on Derivative, Net	(1.0)	(0.9)	(1.7)	(0.9)
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	1.1	(1.1)	0.8	(1.4)
Derivative, Gain (Loss) on Derivative, Net	(1.1)	1.1	(0.8)	1.4
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