Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]
Gross loss and loss expense reserves, beginning of period	$ 6,677	$ 6,088
Less reinsurance recoverable	277	342
Net loss and loss expense reserves, beginning of period	6,400	5,746
Current accident year	1,033	963
Prior accident years	(110)	(33)
Total incurred	923	930
Current accident year	143	186
Prior accident years	562	631
Total paid	705	817
Net loss and loss expense reserves, end of period	6,618	5,859
Plus reinsurance recoverable	262	294
Gross loss and loss expense reserves, end of period	$ 6,880	$ 6,153