Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Feb. 28, 2019
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 6,409	$ 5,954	$ 6,088	$ 5,646
Less reinsurance recoverable	294	269	342	238
Net loss and loss expense reserves, beginning of period	6,115	5,685	5,746	5,408
Current accident year	1,082	916	3,099	2,720
Prior accident years	(11)	(52)	(91)	(203)
Total incurred	1,071	864	3,008	2,517
Current accident year	443	457	998	995
Prior accident years	336	373	1,349	1,457
Total paid	779	830	2,347	2,452
Net loss and loss expense reserves, end of period	6,407	5,719	6,407	5,719
Plus reinsurance recoverable	285	278	285	278
Gross loss and loss expense reserves, end of period	$ 6,692	$ 5,997	$ 6,692	$ 5,997
Cincinnati Global
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Net loss and loss expense reserves related to acquisition of Cincinnati Global at February 28, 2019					$ 246