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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	$ 5,886	$ 5,293	$ 5,646	$ 5,219
Less reinsurance recoverable	266	184	238	187
Net loss and loss expense reserves, beginning of period	5,620	5,109	5,408	5,032
Current accident year	947	852	1,804	1,691
Prior accident years	(84)	(31)	(151)	(79)
Total incurred	863	821	1,653	1,612
Current accident year	361	341	538	536
Prior accident years	437	354	1,084	873
Total paid	798	695	1,622	1,409
Net loss and loss expense reserves, end of period	5,685	5,235	5,685	5,235
Plus reinsurance recoverable	269	188	269	188
Gross loss and loss expense reserves, end of period	$ 5,954	$ 5,423	$ 5,954	$ 5,423
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