Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021		Sep. 30, 2020
Fixed Maturity Securities
Gross Gains on Sales	$ 1.3	$ 3.3	$ 74.9	[1]	$ 8.3
Gross Losses on Sales	(5.4)	(2.8)	(9.0)		(7.6)
Credit Losses	0.0	0.0	(9.3)		(59.3)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	1.5	0.9	5.0		1.2
Gross Losses on Sales	0.0	(0.2)	0.0		(0.4)
Impairment Loss	0.0	8.4	0.0		8.4
Credit Losses	0.0	(1.6)	1.6		(6.2)
Embedded Derivative in Modified Coinsurance Arrangement	2.6	14.1	21.2		(30.9)
All Other Derivatives	0.7	(0.6)	2.3		(2.3)
Foreign Currency Transactions	(0.8)	(0.3)	(1.3)		(0.2)
Net Realized Investment Gain (Loss)	$ (0.1)	$ 4.4	$ 85.4		$ (105.8)

[1] 1Gross gains on sales of fixed maturity securities includes gains of $67.6 million as a result of the second phase of the reinsurance transaction that we completed during the first quarter of 2021. See Note 12 for further discussion.