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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Feb. 28, 2019
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 5,954	$ 5,423	$ 5,646	$ 5,219
Less reinsurance recoverable	269	188	238	187
Net loss and loss expense reserves, beginning of period	5,685	5,235	5,408	5,032
Current accident year	916	857	2,720	2,548
Prior accident years	(52)	(44)	(203)	(123)
Total incurred	864	813	2,517	2,425
Current accident year	457	392	995	928
Prior accident years	373	313	1,457	1,186
Total paid	830	705	2,452	2,114
Net loss and loss expense reserves, end of period	5,719	5,343	5,719	5,343
Plus reinsurance recoverable	278	186	278	186
Gross loss and loss expense reserves, end of period	$ 5,997	$ 5,529	$ 5,997	$ 5,529
Cincinnati Global
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Net loss and loss expense reserves related to acquisition of MSP at February 28, 2019					$ 246