Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fixed Maturity Securities
Gross Gains on Sales	$ 3.3	$ 11.3	$ 8.3	$ 22.5
Gross Losses on Sales	(2.8)	(5.7)	(7.6)	(30.5)
Credit Losses	0.0	(20.8)	(59.3)	(20.8)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0.9	0.6	1.2	4.5
Gross Losses on Sales	(0.2)	0.0	(0.4)	(0.1)
Impairment of Real Estate	(8.4)	0.0	(8.4)	0.0
Credit Losses	(1.6)	0.0	(6.2)	0.0
Embedded Derivative in Modified Coinsurance Arrangement	14.1	(11.1)	(30.9)	(6.4)
All Other Derivatives	(0.6)	(0.1)	(2.3)	(0.7)
Foreign Currency Transactions	(0.3)	(0.4)	(0.2)	(0.9)
Net Realized Investment Gain (Loss)	$ 4.4	$ (26.2)	$ (105.8)	$ (32.4)