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, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 594.7	$ 602.3
Net Realized Investment Gain (Loss)	1.1	(2.2)
Interest and Debt Expense	42.1	40.2
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(0.7)
Interest Rate Swaps | Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	73.0	77.8
Derivative, Gain (Loss) on Derivative, Net	17.7	15.9
Interest Rate Swaps | Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.0	0.7
Derivative, Gain (Loss) on Derivative, Net	0.0	(0.4)
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(3.5)	0.1
Derivative, Gain (Loss) on Derivative, Net	4.1	(0.1)
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(1.5)	1.9
Derivative, Gain (Loss) on Derivative, Net	1.5	(1.9)
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	7.7	11.4
Derivative, Gain (Loss) on Derivative, Net	0.6	0.6
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	3.6	3.6
Derivative, Gain (Loss) on Derivative, Net	0.8	0.1
Foreign Exchange Contracts | Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	4.6	5.0
Derivative, Gain (Loss) on Derivative, Net	(0.1)	(0.3)
Foreign Exchange Contracts | Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.5	0.1
Derivative, Gain (Loss) on Derivative, Net	0.3	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)	0.8	0.0
Derivative, Gain (Loss) on Derivative, Net	(0.8)	0.0
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)	(0.3)
Derivative, Gain (Loss) on Derivative, Net	0.3	0.3
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