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, 2020		Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 593.5		$ 613.2	$ 569.0	$ 585.0	$ 616.3	$ 599.4	$ 624.9	$ 594.7	$ 2,360.7	$ 2,435.3	$ 2,453.7
Net Realized Investment Gain (Loss)	$ 1,304.9	[1]	$ 4.4	$ 33.8	$ (144.0)	$ 9.2	$ (26.2)	$ (7.3)	$ 1.1	1,199.1	(23.2)	(39.5)
Interest and Debt Expense										188.2	177.4	167.3
Derivative, Gain (Loss) on Derivative, Net										2.5	0.1	(0.3)
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income										286.1	294.6	310.3
Derivative, Gain (Loss) on Derivative, Net										75.9	74.3	67.1
Designated as Hedging Instrument | Interest Rate Swaps | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income										0.0	0.0	1.0
Derivative, Gain (Loss) on Derivative, Net										0.0	0.0	(0.5)
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)										397.7	(1.6)	0.1
Derivative, Gain (Loss) on Derivative, Net										32.0	9.3	(0.3)
Designated as Hedging Instrument | Interest Rate Swaps | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)										(0.6)	(4.5)	0.1
Derivative, Gain (Loss) on Derivative, Net										0.6	4.5	(0.1)
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense										29.2	30.4	38.4
Derivative, Gain (Loss) on Derivative, Net										1.7	2.4	2.3
Designated as Hedging Instrument | Interest Rate Swaps | Interest and Debt Expense | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense										10.1	14.3	14.3
Derivative, Gain (Loss) on Derivative, Net										(0.9)	2.5	1.8
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income										12.1	14.8	18.1
Derivative, Gain (Loss) on Derivative, Net										2.5	(2.0)	(0.9)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Investment Income | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income										7.1	2.9	0.6
Derivative, Gain (Loss) on Derivative, Net										2.8	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)										(0.1)	1.4	1.5
Derivative, Gain (Loss) on Derivative, Net										0.1	(1.4)	(1.5)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)										23.3	3.8	(2.7)
Derivative, Gain (Loss) on Derivative, Net										(23.3)	(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	$ 0.0
Derivative, Gain (Loss) on Derivative, Net										$ 0.0

[1] Fourth quarter of 2020 net realized investment gain of $1,302.3 million before tax and $1,028.8 million after tax, related to the transfer of investments for the Closed Block individual disability reinsurance transaction.