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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 623.6	$ 620.5	$ 1,225.9	$ 1,222.9
Net Realized Investment Gain (Loss)	(2.6)	8.1	(4.8)	19.1
Interest and Debt Expense	42.4	39.9	82.6	79.7
Derivative, Gain (Loss) on Derivative, Net	0.0	0.3	(0.7)	0.5
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	76.7	81.0	154.5	162.9
Derivative, Gain (Loss) on Derivative, Net	16.5	15.1	32.4	29.7
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.3	1.5	1.0	2.9
Derivative, Gain (Loss) on Derivative, Net	(0.1)	(1.0)	(0.5)	(2.0)
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	0.0	0.0	0.1	0.2
Derivative, Gain (Loss) on Derivative, Net	(0.2)	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 (Loss)	0.3	(2.0)	2.2	(2.6)
Derivative, Gain (Loss) on Derivative, Net	(0.3)	2.0	(2.2)	2.6
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	11.4	11.4	22.8	22.8
Derivative, Gain (Loss) on Derivative, Net	0.5	0.6	1.1	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	0.5	(0.2)	0.6	(0.5)
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	4.7	5.2	9.7	10.6
Derivative, Gain (Loss) on Derivative, Net	(0.2)	(0.2)	(0.5)	(0.4)
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.3	0.0	0.4	0.0
Derivative, Gain (Loss) on Derivative, Net	0.1	0.0	0.1	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 (Loss)	0.9	0.3	0.9	1.5
Derivative, Gain (Loss) on Derivative, Net	(0.9)	(0.3)	(0.9)	(1.5)
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)	0.0	(1.4)	0.0
Derivative, Gain (Loss) on Derivative, Net	1.1	0.0	1.4	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