Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	$ 5,646	$ 5,219
Less reinsurance recoverable	238	187
Net loss and loss expense reserves, beginning of period	5,408	5,032
Current accident year	857	839
Prior accident years	(67)	(48)
Total incurred	790	791
Current accident year	177	195
Prior accident years	647	519
Total paid	824	714
Net loss and loss expense reserves, end of period	5,620	5,109
Plus reinsurance recoverable	266	184
Gross loss and loss expense reserves, end of period	$ 5,886	$ 5,293
MSP Underwriting Limited
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