Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	$ 6,153	$ 5,886	$ 6,088	$ 5,646
Less reinsurance recoverable	294	266	342	238
Net loss and loss expense reserves, beginning of period	5,859	5,620	5,746	5,408
Current accident year	1,054	947	2,017	1,804
Prior accident years	(47)	(84)	(80)	(151)
Total incurred	1,007	863	1,937	1,653
Current accident year	369	361	555	538
Prior accident years	382	437	1,013	1,084
Total paid	751	798	1,568	1,622
Net loss and loss expense reserves, end of period	6,115	5,685	6,115	5,685
Plus reinsurance recoverable	294	269	294	269
Gross loss and loss expense reserves, end of period	$ 6,409	$ 5,954	$ 6,409	$ 5,954
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