Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2020	[2]	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
Fixed Maturity Securities
Gross Gains on Sales										$ 1,332.8	[1]	$ 22.9	$ 9.7
Gross Losses on Sales										(20.3)		(32.6)	(15.2)
Credit Losses										(53.6)		(25.3)	(17.5)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales										1.9		4.6	0.5
Gross Losses on Sales										(0.3)		(0.3)	0.0
Impairment Loss										(36.6)		0.0	(1.4)
Credit Losses										(4.6)		0.0	0.0
Gain (Loss) on Embedded Derivative										(17.0)		8.3	(15.2)
All Other Derivatives										(2.5)		(0.1)	0.3
Foreign Currency Transactions										(0.7)		(0.7)	(0.7)
Net Realized Investment Gain (Loss)	$ 1,304.9		$ 4.4	$ 33.8	$ (144.0)	$ 9.2	$ (26.2)	$ (7.3)	$ 1.1	$ 1,199.1		$ (23.2)	$ (39.5)

[1] Gross gains on sales of fixed maturity securities includes gains of $1,302.3 million as a result of the reinsurance transaction that we entered into during the fourth quarter of 2020. See Note 12 for further discussion.

[2] 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.