Derivative Financial Instruments Location of Gains and Losses of Derivative Financial Instruments Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 602.3	$ 602.4
Net Realized Investment Gain (Loss)	(2.2)	11.0
Interest and Debt Expense	40.2	39.8
Derivative, Gain (Loss) on Derivative, Net	(0.7)	0.2
Interest Rate Swaps | Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	77.8	81.9
Derivative, Gain (Loss) on Derivative, Net	15.9	14.6
Interest Rate Swaps | Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.7	1.4
Derivative, Gain (Loss) on Derivative, Net	(0.4)	(1.0)
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	0.1	0.2
Derivative, Gain (Loss) on Derivative, Net	(0.1)	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)	1.9	(0.6)
Derivative, Gain (Loss) on Derivative, Net	(1.9)	0.6
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	11.4	11.4
Derivative, Gain (Loss) on Derivative, Net	0.6	0.5
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	3.6	3.6
Derivative, Gain (Loss) on Derivative, Net	0.1	(0.3)
Foreign Exchange Contracts | Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	5.0	5.4
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(0.2)
Foreign Exchange Contracts | Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.1	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	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)	0.0	1.2
Derivative, Gain (Loss) on Derivative, Net	0.0	(1.2)
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(0.3)	0.0
Derivative, Gain (Loss) on Derivative, Net	0.3	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
Derivative, Gain (Loss) on Derivative, Net	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
Derivative, Gain (Loss) on Derivative, Net	$ 0.0	$ 0.0