Schedule of Income subject to domestic and foreign taxation (Detail) - USD ($) $ in Millions	3 Months Ended										12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	[2]	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Abstract]
Income Before Tax United States - Federal and State											$ 1,289.0	$ 1,215.8	$ 1,057.8
Income Before Tax Foreign											115.0	131.9	180.5
Income Before Income Tax	$ 345.3		$ 366.8	$ 361.6	$ 330.3		$ 371.2	$ 332.1	$ 340.2	$ 304.2	1,404.0	1,347.7	1,238.3
Current Tax Expense United States - Federal											374.9	414.8	280.5
Current State and Local Tax Expense (Benefit)											0.4	0.3	0.0
Current Tax Expense Foreign											26.0	(29.4)	61.6
Income Tax - Current											401.3	385.7	342.1
Deferred Tax Expense (Benefit) United States - Federal											14.2	(14.6)	56.9
Deferred State and Local Income Tax Expense (Benefit)											(0.3)	(2.1)	0.0
Deferred Tax Expense (Benefit) Foreign											(5.4)	47.3	(27.8)
Income Tax - Deferred											8.5	30.6	29.1
Income Tax Expense											$ 409.8	$ 416.3	$ 371.2

[1] Fourth quarter of 2017 reserve increase of $39.0 million before tax and $25.4 million after tax related to unclaimed death benefits.

[2] First quarter of 2017 loss from a guaranty fund assessment related to an unaffiliated insurer that was declared insolvent of $20.6 million before tax and $13.4 million after tax.