Schedule of Income subject to domestic and foreign taxation (Detail) - USD ($) $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2019	[1]	Sep. 30, 2019	[2]	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	[3]	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax [Abstract]
Income Before Tax, Domestic												$ 1,289.5	$ 492.6	$ 1,289.0
Income Before Tax, Foreign												92.6	135.2	115.0
Income Before Income Tax	$ 377.4		$ 299.4		$ 352.0	$ 353.3	$ 309.7	$ (377.8)		$ 354.8	$ 341.1	1,382.1	627.8	1,404.0
Current Tax Expense, Domestic												273.6	194.6	374.9
Current State and Local Tax Expense (Benefit)												1.3	(0.6)	0.4
Current Tax Expense (Benefit), Foreign												(0.1)	33.4	26.0
Income Tax - Current												274.8	227.4	401.3
Deferred Tax Expense (Benefit), Federal												(9.5)	(114.6)	14.2
Deferred State and Local Income Tax Expense (Benefit)												(0.1)	(0.2)	(0.3)
Deferred Tax Expense (Benefit), Foreign												16.6	(8.2)	(5.4)
Deferred Income Tax Expense (Benefit)												7.0	(123.0)	8.5
Income Tax Expense												$ 281.8	$ 104.4	$ 409.8

[1] Fourth quarter of 2019 cost related to the early retirement of debt of $2.1 million before tax and $1.7 million after tax

[2] Third quarter of 2019 cost related to the early retirement of debt of $25.2 million before tax and $19.9 million after tax.

[3] Third quarter of 2018 reserve increase of $750.8 million before tax and $593.1 million after tax related to long-term care.