Derivative Financial Instruments Location of Gains and Losses Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 608.6	$ 619.2	$ 623.6	$ 602.3	$ 619.8	$ 609.0	$ 620.5	$ 602.4	$ 2,453.7	$ 2,451.7	$ 2,459.0
Net Realized Investment Gain (Loss)	$ (41.4)	$ 6.7	$ (2.6)	$ (2.2)	$ 11.4	$ 9.8	$ 8.1	$ 11.0	(39.5)	40.3	24.2
Interest and Debt Expense									167.3	159.9	166.0
Derivative, Gain (Loss) on Derivative, Net									(0.3)	0.1	1.5
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income									310.3	321.1	337.7
Derivative, Gain (Loss) on Derivative, Net									67.1	60.4	55.7
Interest Rate Swaps | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income									1.0	4.8	7.1
Derivative, Gain (Loss) on Derivative, Net									(0.5)	(3.2)	(5.3)
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	2.5	7.2
Derivative, Gain (Loss) on Derivative, Net									(0.3)	0.2	3.4
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)									0.1	(1.7)	(3.1)
Derivative, Gain (Loss) on Derivative, Net									(0.1)	1.7	3.1
Interest Rate Swaps | Interest and Debt Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense									38.4	44.9	44.9
Derivative, Gain (Loss) on Derivative, Net									2.3	2.2	2.1
Interest Rate Swaps | Interest and Debt Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense									14.3	14.3	39.7
Derivative, Gain (Loss) on Derivative, Net									1.8	(0.6)	(4.7)
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income									18.1	21.7	23.1
Derivative, Gain (Loss) on Derivative, Net									(0.9)	(1.2)	(0.6)
Foreign Exchange Contracts | Net Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income									0.6
Derivative, Gain (Loss) on Derivative, Net									0.5
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)									1.5	8.9	1.5
Derivative, Gain (Loss) on Derivative, Net									(1.5)	(9.2)	(1.4)
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)									(2.7)
Derivative, Gain (Loss) on Derivative, Net									2.7
Foreign Exchange Contracts | Interest and Debt Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense									0.0	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net									0.0	$ 0.0	$ 0.0
Foreign Exchange Contracts | Interest and Debt Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense									0.0
Derivative, Gain (Loss) on Derivative, Net									$ 0.0