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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 619.2	$ 609.0	$ 1,845.1	$ 1,831.9
Net Realized Investment Gain	6.7	9.8	1.9	28.9
Interest and Debt Expense	42.7	40.1	125.3	119.8
Derivative, Gain (Loss) on Derivative, Net	(0.2)	(0.2)	(0.9)	0.3
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	75.8	79.7	230.3	242.6
Derivative, Gain (Loss) on Derivative, Net	16.7	15.6	49.1	45.3
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.0	1.2	1.0	4.1
Derivative, Gain (Loss) on Derivative, Net	0.0	(0.8)	(0.5)	(2.8)
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain	0.0	0.0	0.1	0.2
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	(0.3)	0.0
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain	0.0	(0.5)	2.2	(3.1)
Derivative, Gain (Loss) on Derivative, Net	0.0	0.5	(2.2)	3.1
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	8.5	11.4	31.3	34.2
Derivative, Gain (Loss) on Derivative, Net	0.6	0.5	1.7	1.6
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	3.6	3.6	10.8	10.8
Derivative, Gain (Loss) on Derivative, Net	0.6	(0.1)	1.2	(0.6)
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	4.6	6.5	14.3	17.1
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(0.5)	(0.8)	(0.9)
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.1	0.0	0.5	0.0
Derivative, Gain (Loss) on Derivative, Net	0.1	0.0	0.2	0.0
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain	0.4	5.2	1.3	6.7
Derivative, Gain (Loss) on Derivative, Net	(0.4)	(5.2)	(1.3)	(6.7)
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain	(0.3)	0.0	(1.7)	0.0
Derivative, Gain (Loss) on Derivative, Net	0.3	0.0	1.7	0.0
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