Schedule V - Supplemental Information Concerning Property and Casualty Insurance Operations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred acquisition costs	$ 662	$ 633
Reserves for unpaid claim and claim adjustment expenses	21,720	21,984
Discount deducted from claim and claim adjustment expense reserves above (based on interest rates ranging from 3.5% to 7.6%)	1,321	1,388
Unearned premiums	4,583	4,183
Net written premiums	7,656	7,345	$ 7,069
Net earned premiums	7,428	7,312	6,988
Net investment income	2,063	1,751	1,992
Incurred claim and claim adjustment expenses related to current year	5,356	5,358	5,201
Incurred claim and claim adjustment expenses related to prior years	(127)	(179)	(381)
Amortization of deferred acquisition costs	1,383	1,335	1,233
Paid claim and claim adjustment expenses	$ 5,576	$ 5,331	$ 5,341