Schedule V - Supplemental Information Concerning Property and Casualty Insurance Operations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred acquisition costs	$ 632	$ 632
Reserves for unpaid claim and claim adjustment expenses	21,984	22,004
Discount deducted from claim and claim adjustment expense reserves above (based on interest rates ranging from 3.5% to 8.0%)	1,388	1,434
Unearned premiums	4,183	4,029
Net written premiums	7,345	7,069	$ 6,988
Net earned premiums	7,312	6,988	6,924
Net investment income	1,751	1,992	1,952
Incurred claim and claim adjustment expenses related to current year	5,358	5,201	5,025
Incurred claim and claim adjustment expenses related to prior years	(179)	(381)	(342)
Amortization of deferred acquisition costs	1,335	1,233	1,235
Paid claim and claim adjustment expenses	$ 5,331	$ 5,341	$ 5,134