Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Fixed Maturity Securities
Gross Gains on Sales	$ 3.6	$ 7.9	$ 5.0	$ 11.2
Gross Losses on Sales	(4.1)	(16.9)	(4.8)	(24.8)
Credit Losses	(5.4)	0.0	(59.3)	0.0
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0.0	3.0	0.3	3.9
Gross Losses on Sales	0.0	0.0	(0.2)	(0.1)
Credit Losses	(2.0)	0.0	(4.6)	0.0
Embedded Derivative in Modified Coinsurance Arrangement	41.9	(0.8)	(45.0)	4.7
All Other Derivatives	(1.0)	(0.9)	(1.7)	(0.6)
Foreign Currency Transactions	0.8	0.4	0.1	(0.5)
Net Realized Investment Gain (Loss)	$ 33.8	$ (7.3)	$ (110.2)	$ (6.2)