Liability for Unpaid Claims and Claims Adjustment Expense (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense
Balance at January 1	$ 24,180.2	$ 23,076.7
Less Reinsurance Recoverable	8,378.9	2,246.8
Net Balance at January 1	15,801.3	20,829.9
Incurred Related to
Current Year	3,461.0	3,187.2
Paid Related to
Current Year	(1,151.3)	(1,015.9)
Prior Years	(2,215.8)	(2,546.6)
Total Paid	(3,367.1)	(3,562.5)
Reserves Ceded Pursuant to Reinsurance Transaction	(990.0)	0.0
Net Balance at June 30	15,072.1	20,582.4
Plus Reinsurance Recoverable	9,042.2	2,260.2
Balance at June 30	24,114.3	22,842.6
Amount Related to Interest
Incurred Related to
Prior Years	359.2	528.5
Incurred Claims
Incurred Related to
Prior Years	(217.3)	(270.5)
Foreign Currency
Incurred Related to
Prior Years	25.0	(130.2)
Operating Expense
Incurred Related to
Total Incurred	$ 3,627.9	$ 3,315.0