Liability for Unpaid Claims and Claims Adjustment Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense
Balance at January 1	$ 23,249.5	$ 23,796.1	$ 24,194.0
Less Reinsurance Recoverable	2,163.6	2,064.6	2,066.9
Net Balance at January 1	21,085.9	21,731.5	22,127.1
Incurred Related to
Current Year	5,422.9	5,243.0	5,058.1
Paid Related to
Current Year	(2,156.9)	(2,035.4)	(1,853.7)
Prior Years	(4,410.9)	(4,451.4)	(4,546.5)
Total Paid	(6,567.8)	(6,486.8)	(6,400.2)
Net Balance at December 31	21,040.0	21,085.9	21,731.5
Plus Reinsurance Recoverable	2,182.0	2,163.6	2,064.6
Balance at December 31	23,222.0	23,249.5	23,796.1
Amount Related to Interest
Incurred Related to
Prior Years	1,089.7	1,136.7	1,177.6
Incurred Claims
Incurred Related to
Prior Years	(158.2)	(198.3)	(111.6)
Foreign Currency
Incurred Related to
Prior Years	167.5	(340.2)	(119.5)
Operating Expense
Incurred Related to
Total Incurred	$ 6,521.9	$ 5,841.2	$ 6,004.6