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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 616.3	$ 599.4	$ 624.9	$ 594.7	$ 608.6	$ 619.2	$ 623.6	$ 602.3	$ 2,435.3	$ 2,453.7	$ 2,451.7
Net Realized Investment Gain (Loss)	$ 9.2	$ (26.2)	$ (7.3)	$ 1.1	$ (41.4)	$ 6.7	$ (2.6)	$ (2.2)	(23.2)	(39.5)	40.3
Interest and Debt Expense									177.4	167.3	159.9
Derivative, Gain (Loss) on Derivative, Net									0.1	(0.3)	0.1
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income									294.6	310.3	321.1
Derivative, Gain (Loss) on Derivative, Net									74.3	67.1	60.4
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income									0.0	1.0	4.8
Derivative, Gain (Loss) on Derivative, Net									0.0	(0.5)	(3.2)
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)									(1.6)	0.1	2.5
Derivative, Gain (Loss) on Derivative, Net									9.3	(0.3)	0.2
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)									(4.5)	0.1	(1.7)
Derivative, Gain (Loss) on Derivative, Net									4.5	(0.1)	1.7
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense									30.4	38.4	44.9
Derivative, Gain (Loss) on Derivative, Net									2.4	2.3	2.2
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense									14.3	14.3	14.3
Derivative, Gain (Loss) on Derivative, Net									2.5	1.8	(0.6)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income									14.8	18.1	21.7
Derivative, Gain (Loss) on Derivative, Net									(2.0)	(0.9)	(1.2)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income									2.9	0.6
Derivative, Gain (Loss) on Derivative, Net									1.9	0.5
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)									1.4	1.5	8.9
Derivative, Gain (Loss) on Derivative, Net									(1.4)	(1.5)	(9.2)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)									3.8	(2.7)
Derivative, Gain (Loss) on Derivative, Net									(3.8)	2.7
Designated as Hedging Instrument | Foreign Exchange Contracts | Interest and Debt Expense | 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
Designated as Hedging Instrument | Foreign Exchange Contracts | Interest and Debt Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense									0.0	$ 0.0
Derivative, Gain (Loss) on Derivative, Net									$ 0.0