Liability for Unpaid Claims and Claims Adjustment Expense (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense
Balance at January 1	$ 23,664.7	$ 24,180.2
Less Reinsurance Recoverable	8,697.8	8,378.9
Net Balance at January 1	14,966.9	15,801.3
Incurred Related to
Current Year	1,856.6	1,870.3
Paid Related to
Current Year	(483.7)	(472.1)
Prior Years	(1,337.1)	(1,342.5)
Total Paid	(1,820.8)	(1,814.6)
Reserves Ceded Pursuant to Reinsurance Transaction	0.0	(990.0)
Net Balance at March 31	14,870.9	15,001.7
Plus Reinsurance Recoverable	8,535.3	9,225.8
Balance at March 31	23,406.2	24,227.5
Amount Related to Interest
Incurred Related to
Prior Years	164.0	183.4
Incurred Claims
Incurred Related to
Prior Years	(231.8)	(66.0)
Foreign Currency
Incurred Related to
Prior Years	(64.0)	17.3
Operating Expense
Incurred Related to
Total Incurred	$ 1,724.8	$ 2,005.0