Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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	$ 6,088	$ 5,646
Less reinsurance recoverable	342	238
Net loss and loss expense reserves, beginning of period	5,746	5,408
Current accident year	963	857
Prior accident years	(33)	(67)
Total incurred	930	790
Current accident year	186	177
Prior accident years	631	647
Total paid	817	824
Net loss and loss expense reserves, end of period	5,859	5,620
Plus reinsurance recoverable	294	266
Gross loss and loss expense reserves, end of period	$ 6,153	$ 5,886
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